Introduction

When Netscape started the Mozilla browser, it made the conscious decision to support W3C standards. As a result, Mozilla is not fully backwards-compatible with Netscape Navigator 4.x and Microsoft Internet Explorer legacy code; for example, Mozilla does not support <layer> as I will discuss later. Browsers, like Internet Explorer 4, that were built before the conception of W3C standards inherited many quirks. In this article, I will describe Mozilla's quirks mode, which provides strong backwards HTML compatibility with Internet Explorer and other legacy browsers.

I'll also cover nonstandard technologies, such as XMLHttpRequest and rich text editing, that Mozilla does support because no W3C equivalent existed at the time. They include:

• HTML 4.01, XHTML 1.0 and XHTML 1.1
• Cascade Style Sheets (CSS): CSS Level 1, CSS Level 2.1 and parts of CSS Level 3
• Document Object Model (DOM): DOM Level 1, DOM Level 2 and parts of DOM Level 3
• Mathematical Markup Language: MathML Version 2.0
• Extensible Markup Language (XML): XML 1.0, Namespaces in XML, Associating Style Sheets with XML Documents 1.0, Fragment Identifier for XML
• XSL Transformations: XSLT 1.0
• XML Path Language: XPath 1.0
• Resource Description Framework: RDF
• Simple Object Access Protocol: SOAP 1.1
• ECMA-262, revision 3 (JavaScript 1.5): ECMA-262

General cross-browser coding tips

Even though Web standards do exist, different browsers behave differently (in fact, the same browser may behave differently depending on the platform). Many browsers, such as Internet Explorer, also support pre-W3C APIs and have never added extensive support for the W3C-compliant ones.

Before I go into the differences between Mozilla and Internet Explorer, I'll cover some basic ways you can make a Web application extensible in order to add new browser support later.

Since different browsers sometimes use different APIs for the same functionality, you can often find multiple if() else() blocks throughout the code to differentiate between the browsers. The following code shows blocks designated for Internet Explorer:

. . .

var elm;

if (ns4)
  elm = document.layers["myID"];
else if (ie4)
  elm = document.all["myID"]

The above code isn't extensible, so if you want it to support a new browser, you must update these blocks throughout the Web application.

The easiest way to eliminate the need to recode for a new browser is to abstract out functionality. Rather than multiple if() else() blocks, you increase efficiency by taking common tasks and abstracting them out into their own functions. Not only does this make the code easier to read, it simplifies adding support for new clients:

var elm = getElmById("myID");

function getElmById(aID){
  var element = null;

  if (isMozilla || isIE5)
    element = document.getElementById(aID);
  else if (isNetscape4)
    element = document.layers[aID];
  else if (isIE4)
    element = document.all[aID];

  return element;
}

The above code still has the issue of browser sniffing, or detecting which browser the user is using. Browser sniffing is usually done through the useragent, such as:

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.5) Gecko/20031016

While using the useragent to sniff the browser provides detailed information on the browser in use, code that handles useragents often can make mistakes when new browser versions arrive, thus requiring code changes.

It is much better and more reliable to sniff by browser capability or object feature support. You can usually do this by testing the required functionality in JavaScript. For example, rather than:

if (isMozilla || isIE5)

You would use:

if (document.getElementById)

This would allow other browsers that support that W3C standard method, such as Opera or Safari, to work without any changes.

Useragent sniffing makes sense in very limited cases where you really do want to know the exact browser in use, such as when  you need to work around a bug in a specific version of a specific browser.  For all other purposes you should check for the feature you need rather than the browsers you think will support that feature.

JavaScript also allows inline conditional statements, which can help with code readability:

var foo = (condition) ? conditionIsTrue : conditionIsFalse;

For example, to retrieve an element, you would use:

function getElement(aID){
  return (document.getElementById) ? document.getElementById(aID)
                                   : document.all[aID]);
}

Differences between Mozilla and Internet Explorer

First, I'll discuss the differences in the way HTML behaves between Mozilla and Internet Explorer.

Tooltips

Legacy browsers introduced tooltips into HTML by showing them on links and using the value of the alt attribute as a tooltip's content. The latest W3C HTML specification created the title attribute, which is meant to contain a detailed description of the link. Modern browsers will use the title attribute to display tooltips, and Mozilla only supports showing tooltips for that attribute and not the alt attribute.

Entities

HTML markup can contain several entities, which the W3C web standards body has defined. You can reference entities through their numerical or character reference. For example, you can reference the non-breaking space character with   or with its equivalent character reference  .

Some older browsers, such as Internet Explorer, had such quirks as allowing you to use entities by replacing the ; (semi-colon) character at the end with regular text content:

&nbsp Foo
&nbsp&nbsp Foo

Internet Explorer will render the above &nbsp as white spaces, even though that is against the W3C specification. The browser will not parse a &nbsp if it is directly followed by more characters, for example:

&nbsp12345

This code does not work in Mozilla, since it goes against the W3C web standards. Always use the correct form ( ) to avoid browser discrepancies.

DOM differences

The Document Object Model (DOM) is the tree structure that contains the document elements. You can manipulate it through JavaScript APIs, which the W3C has standardized. However, prior to W3C standardization, Netscape 4 and Internet Explorer 4 implemented the APIs similarly. Mozilla only implements legacy APIs if they are unachievable with W3C web standards.

Accessing elements

To retrieve an element reference using the cross-browser approach, you use document.getElementById(aID), which works in Internet Explorer 5.0+, Mozilla-based browsers, other W3C-compliant browsers and is part of the DOM Level 1 specification.

Mozilla does not support accessing an element through document.elementName or even through the element's name, which Internet Explorer does (also called global namespace polluting). Mozilla also does not support the Netscape 4 document.layers method and Internet Explorer's document.all. While document.getElementById lets you retrieve one element, you can also use document.layers and document.all to obtain a list of all document elements with a certain tag name, such as all <div> elements.

The W3C DOM Level 1 method gets references to all elements with the same tag name through getElementsByTagName(). The method returns an array in JavaScript, and can be called on the document element or other nodes to search only their subtree. To get an array of all elements in the DOM tree, you can use getElementsByTagName("*").

The DOM Level 1 methods, as shown in Table 1, are commonly used to move an element to a certain position and toggle its visibility (menus, animations). Netscape 4 used the <layer> tag, which Mozilla does not support, as an HTML element that can be positioned anywhere. In Mozilla, you can position any element using the <div> tag, which Internet Explorer uses as well and which you'll find in the HTML specification.

Traverse the DOM

Mozilla supports the W3C DOM APIs for traversing the DOM tree through JavaScript (see Table 2). The APIs exist for each node in the document and allow walking the tree in any direction. Internet Explorer supports these APIs as well, but it also supports its legacy APIs for walking a DOM tree, such as the children property.

Internet Explorer has a nonstandard quirk, where many of these APIs will skip white space text nodes that are generated, for example, by new line characters. Mozilla will not skip these, so sometimes you need to distinguish these nodes. Every node has a nodeType property specifying the node type. For example, an element node has type 1, while a text node has type 3 and a comment node is type 8. The best way to only process element nodes is to iterate over all child nodes and only process those with a nodeType of 1:

HTML:
  <div id="foo">
    <span>Test</span>
  </div>

JavaScript:
  var myDiv = document.getElementById("foo");
  var myChildren = myDiv.childNodes;
  for (var i = 0; i < myChildren.length; i++) {
    if (myChildren[i].nodeType == 1){
      // element node
    };
  };

Generate and manipulate content

Mozilla supports the legacy methods for adding content into the DOM dynamically, such as document.write, document.open and document.close. Mozilla also supports Internet Explorer's innerHTML method, which it can call on almost any node. It does not, however, support outerHTML (which adds markup around an element, and has no standard equivalent) and innerText (which sets the text value of the node, and which you can achieve in Mozilla by using textContent).

Internet Explorer has several content manipulation methods that are nonstandard and unsupported in Mozilla, including retrieving the value, inserting text and inserting elements adjacent to a node, such as getAdjacentElement and insertAdjacentHTML. Table 3 shows how the W3C standard and Mozilla manipulate content, all of which are methods of any DOM node.

Document fragments

For performance reasons, you can create documents in memory, rather than working on the existing document's DOM. DOM Level 1 Core introduced document fragments, which are lightweight documents that contain a subset of a normal document's interfaces. For example, getElementById does not exist, but appendChild does. You can also easily add document fragments to existing documents.

Mozilla creates document fragments through document.createDocumentFragment(), which returns an empty document fragment.

Internet Explorer's implementation of document fragments, however, does not comply with the W3C web standards and simply returns a regular document.

JavaScript differences

Most differences between Mozilla and Internet Explorer are usually blamed on JavaScript. However, the issues usually lie in the APIs that a browser exposes to JavaScript, such as the DOM hooks. The two browsers possess few core JavaScript differences; issues encountered are often timing related.

JavaScript date differences

The only Date difference is the getYear method. As per the ECMAScript specification (which is the specification JavaScript follows), running new Date().getYear() in 2004 will return "104". Per the ECMAScript specification, getYear returns the year minus 1900, originally meant to return "98" for 1998. getYear was deprecated in ECMAScript Version 3 and replaced with getFullYear(). Internet Explorer uses an obsolete behavior for getYear(), similar to Netscape 4 and earlier, returning the year minus 1900 for years 1900-1999, and the full year for any other year. (i.e. for 1999, getYear() returns 99, for 2000, it returns 2000).

JavaScript execution differences

Different browsers execute JavaScript differently. For example, the following code assumes that the div node already exists in the DOM by the time the script block executes:

...
<div id="foo">Loading...</div>

<script>
  document.getElementById("foo").innerHTML = "Done.";
</script>

However, this is not guaranteed. To be sure that all elements exist, you should use the onload event handler on the <body> tag:

<body onload="doFinish();">

<div id="foo">Loading...</div>

<script>
  function doFinish() {
    var element = document.getElementById("foo");
    element.innerHTML = "Done.";
  }
</script>
...

Such timing-related issues are also hardware-related -- slower systems can reveal bugs that faster systems hide. One concrete example is window.open, which opens a new window:

<script>
  function doOpenWindow(){
    var myWindow = window.open("about:blank");
    myWindow.location.href = "http://www.ibm.com";
  }
</script>

The problem with the code is that window.open is asynchronous -- it does not block the JavaScript execution until the window has finished loading. Therefore, you may execute the line after the window.open line before the new window has finished. You can deal with this by having an onload handler in the new window and then call back into the opener window (using window.opener).

Differences in JavaScript-generating HTML

JavaScript can, through document.write, generate HTML on the fly from a string. The main issue here is when JavaScript, embedded inside an HTML document (thus, inside an <script> tag), generates HTML that contains a <script> tag. If the document is in strict rendering mode, it will parse the </script> inside the string as the closing tag for the enclosing <script>. The following code illustrates this best:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
...
<script>
  document.write("<script type='text/javascript'>alert('Hello');</script>")
</script>

Since the page is in strict mode, Mozilla's parser will see the first <script> and parse until it finds a closing tag for it, which would be the first </script>. This is because the parser has no knowledge about JavaScript (or any other language) when in strict mode. In quirks mode, the parser is aware of JavaScript when parsing (which slows it down). Internet Explorer is always in quirks mode, as it does not support true XHTML. To make this work in strict mode in Mozilla, separate the string into two parts:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
...
<script>
  document.write("<script type='text\/javascript'>alert('Hello');</" + "script>")
</script>

Debug JavaScript

Mozilla provides several ways to debug JavaScript-related issues found in applications created for Internet Explorer. The first tool is the built-in JavaScript console, shown in Figure 1, where errors and warnings are logged. You can access it in Mozilla by going to Tools -> Web Development -> JavaScript Console or in Firefox (the standalone browser product from Mozilla) at Tools -> JavaScript Console.

Figure 1. JavaScript console

The JavaScript console can show the full log list or just errors, warnings, and messages. The error message in Figure 1 says that at aol.com, line 95 tries to access an undefined variable called is_ns70. Clicking on the link will open Mozilla's internal view source window with the offending line highlighted.

The console also allows you to evaluate JavaScript. To evaluate the entered JavaScript syntax, type in 1+1 into the input field and press Evaluate, as Figure 2 shows.

Figure 2. JavaScript console evaluating

Mozilla's JavaScript engine has built-in support for debugging, and thus can provide powerful tools for JavaScript developers. Venkman, shown in Figure 3, is a powerful, cross-platform JavaScript debugger that integrates with Mozilla. It is usually bundled with Mozilla releases; you can find it at Tools -> Web Development -> JavaScript Debugger. For Firefox, the debugger isn't bundled; instead, you can download and install it from the Venkman Project Page. You can also find tutorials at the development page, located at the Venkman Development Page.

Figure 3. Mozilla's JavaScript debugger

The JavaScript debugger can debug JavaScript running in the Mozilla browser window. It supports such standard debugging features as breakpoint management, call stack inspection, and variable/object inspection. All features are accessible through the user interface or through the debugger's interactive console. With the console, you can execute arbitrary JavaScript in the same scope as the JavaScript currently being debugged.

CSS differences

Mozilla-based products have the strongest support for Cascading Style Sheets (CSS), including most of CSS1, CSS2.1 and parts of CSS3, compared to Internet Explorer as well as all other browsers.

For most issues mentioned below, Mozilla will add an error or warning entry into the JavaScript console. Check the JavaScript console if you encounter CSS-related issues.

Mimetypes (when CSS files are not applied)

The most common CSS-related issue is that CSS definitions inside referenced CSS files are not applied. This is usually due to the server sending the wrong mimetype for the CSS file. The CSS specification states that CSS files should be served with the text/css mimetype. Mozilla will respect this and only load CSS files with that mimetype if the Web page is in strict standards mode. Internet Explorer will always load the CSS file, no matter with which mimetype it is served. Web pages are considered in strict standards mode when they start with a strict doctype. To solve this problem, you can make the server send the right mimetype or remove the doctype. I'll discuss more about doctypes in the next section.

CSS and units

Many Web applications do not use units with their CSS, especially when you use JavaScript to set the CSS. Mozilla tolerates this, as long as the page is not rendered in strict mode. Since Internet Explorer does not support true XHTML, it does not care if no units are specified. If the page is in strict standards mode, and no units are used, then Mozilla ignores the style:

<DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
  <head>
   <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
   <title>CSS and units example</title>
  </head>
  <body>
    // works in strict mode
    <div style="width: 40px; border: 1px solid black;">
      Text
    </div>

    // will fail in strict mode
    <div style="width: 40; border: 1px solid black;">
      Text
    </div>
  </body>
</html>

Since the above example has a strict doctype, the page is rendered in strict standards mode. The first div will have a width of 40px, since it uses units, but the second div won't get a width, and thus will default to 100% width. The same would apply if the width were set through JavaScript.

JavaScript and CSS

Since Mozilla supports the CSS standards, it also supports the CSS DOM standard for setting CSS through JavaScript. You can access, remove, and change an element's CSS rules through the element's style member:

<div id="myDiv" style="border: _fckstyle="border: 1px solid black;">
  Text
</div>

<script>
  var myElm = document.getElementById("myDiv");
  myElm.style.width = "40px";
</script>

You can reach every CSS attribute that way. Again, if the Web page is in strict mode, you must set a unit or else Mozilla will ignore the command. When you query a value, say through .style.width, in Mozilla and Internet Explorer, the returned value will contain the unit, meaning a string is returned. You can convert the string into a number through parseFloat("40px").

CSS overflow differences

CSS added the notion of overflow, which allows you to define how to handle overflow; for example, when the contents of a div with a specified height are taller than that height. The CSS standard defines that if no overflow behavior is set in this case, the div contents will overflow. However, Internet Explorer does not comply with this and will expand the div beyond its set height in order to hold the contents. Below is an example that shows this difference:

<div style="height: _fckstyle="height: 100px; border: 1px solid black;">
  <div style="height: 150px; border: 1px solid red; margin: 10px;">
    a
  </div>
</div>

As you can see in Figure 4, Mozilla acts like the W3C standard specifies. The W3C standard says that, in this case, the inner div overflows to the bottom since the inner content is taller than its parent. If you prefer the Internet Explorer behavior, simply do not specify a height on the outer element.

Figure 4. DIV overflow

hover differences

The nonstandard CSS hover behavior in Internet Explorer occurs on quite a few web sites. It usually manifests itself by changing text style when hovered over in Mozilla, but not in Internet Explorer. This is because the a:hover CSS selector in Internet Explorer matches <a href="">...</a> but not <a name="">...</a>, which sets anchors in HTML. The text changes occur because authors encapsulate the areas with the anchor-setting markup:

CSS:
  a:hover {color: green;}

HTML:
  <a href="foo.com">This text should turn green when you hover over it.</a>

  <a name="anchor-name">
    This text should change color when hovered over, but doesn't
    in Internet Explorer.
  </a>

Mozilla follows the CSS specification correctly and will change the color to green in this example. You can use two ways to make Mozilla behave like Internet Explorer and not change the color of the text when hovered over:

• First, you can change the CSS rule to be a:link:hover {color: green;}, which will only change the color if the element is a link (has an href attribute).
• Alternatively, you can change the markup and close the opened <a /> before the start of the text -- the anchor will continue to work this way.

Quirks versus standards mode

Older legacy browsers, such as Internet Explorer 4, rendered with so-called quirks under certain conditions. While Mozilla aims to be a standards-compliant browser, it has three modes that support older Web pages created with these quirky behaviors. The page's content and delivery determine which mode Mozilla will use. Mozilla will indicate the rendering mode in View -> Page Info (or Ctrl+I) ; Firefox will list the rendering mode in Tools -> Page Info. The mode in which a page is located depends on its doctype.

Doctypes (short for document type declarations) look like this:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

The section in blue is called the public identifier, the green part is the system identifier, which is a URI.

Standards mode

Standards mode is the strictest rendering mode -- it will render pages per the W3C HTML and CSS specifications and will not support any quirks. Mozilla uses it for the following conditions:

• If a page is sent with a text/xml mimetype or any other XML or XHTML mimetype
• For any "DOCTYPE HTML SYSTEM" doctype (for example, <!DOCTYPE HTML SYSTEM "http://www.w3.org/TR/REC-html40/strict.dtd">), except for the IBM doctype
• For unknown doctypes or doctypes without DTDs

Almost standards mode

Mozilla introduced almost standards mode for one reason: a section in the CSS 2 specification breaks designs based on a precise layout of small images in table cells. Instead of forming one image to the user, each small image ends up with a gap next to it. The old IBM homepage shown in Figure 5 offers an example.

Figure 5. Image gap

Almost standards mode behaves almost exactly as standards mode, except when it comes to an image gap issue. The issue occurs often on standards-compliant pages and causes them to display incorrectly.

Mozilla uses almost standards mode for the following conditions:

• For any "loose" doctype (for example, <!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN">, <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">)
• For the IBM doctype (<!DOCTYPE html SYSTEM "http://www.ibm.com/data/dtd/v11/ibmxhtml1-transitional.dtd">)

You can read more about the image gap issue.

Quirks mode

Currently, the Web is full of invalid HTML markup, as well as markup that only functions due to bugs in browsers. The old Netscape browsers, when they were the market leaders, had bugs. When Internet Explorer arrived, it mimicked those bugs in order to work with the content at that time. As newer browsers came to market, most of these original bugs, usually called quirks, were kept for backwards compatibility. Mozilla supports many of these in its quirks rendering mode. Note that due to these quirks, pages will render slower than if they were fully standards-compliant. Most Web pages are rendered under this mode.

Mozilla uses quirks mode for the following conditions:

• When no doctype is specified
• For doctypes without a system identifier (for example, <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">)

For further reading, check out: Mozilla Quirks Mode Behavior and Mozilla's DOCTYPE sniffing.

Event differences

Mozilla and Internet Explorer are almost completely different in the area of events. The Mozilla event model follows the W3C and Netscape model. In Internet Explorer, if a function is called from an event, it can access the event object through window.event. Mozilla passes an event object to event handlers. They must specifically pass the object on to the function called through an argument.

A cross-browser event handling example follows (note that it means you can't define a global variable named event in your code):

<div onclick="handleEvent(event);">Click me!</div>

<script>
  function handleEvent(aEvent) {
    var myEvent = window.event ? window.event : aEvent;
  }
</script>

The properties and functions that the event object exposes are also often named differently in Mozilla and Internet Explorer, as Table 4 shows.

Attach event handlers

Mozilla supports two ways to attach events through JavaScript. The first, supported by all browsers, sets event properties directly on objects. To set a click event handler, a function reference is passed to the object's onclick property:

<div id="myDiv">Click me!</div>

<script>
  function handleEvent(aEvent) {
    // if aEvent is null, means the Internet Explorer event model,
    // so get window.event.
    var myEvent = aEvent ? aEvent : window.event;
  }

  function onPageLoad(){
    document.getElementById("myDiv").onclick = handleEvent;
  }
</script>

Mozilla fully supports the W3C standard way of attaching listeners to DOM nodes. You use the addEventListener() and removeEventListener() methods, and have the benefit of being able to set multiple listeners for the same event type. Both methods require three parameters: the event type, a function reference, and a boolean denoting whether the listener should catch events in their capture phase. If the boolean is set to false, it will only catch bubbling events. W3C events have three phases: capturing, at target, and bubbling. Every event object has an eventPhase attribute indicating the phase numerically (0 indexed). Every time you trigger an event, the event starts at the DOM's outermost element, the element at the top of the DOM tree. It then walks the DOM using the most direct route toward the target, which is the capturing phase. When the event reaches the target, the event is in the target phase. After arriving at the target, it walks up the DOM tree back to the outermost node; this is bubbling. Internet Explorer's event model only has the bubbling phase; therefore, setting the third parameter to false results in Internet Explorer-like behavior:

<div id="myDiv">Click me!</div>

<script>

  function handleEvent(aEvent) {
    // if aEvent is null, it is the Internet Explorer event model,
    // so get window.event.
    var myEvent = aEvent ? aEvent : window.event;
  }

  function onPageLoad() {
    var element = document.getElementById("myDiv");
    element.addEventListener("click", handleEvent, false);
  }
</script>

One advantage of addEventListener() and removeEventListener() over setting properties is that you can have multiple event listeners for the same event, each calling another function. Thus, to remove an event listener requires all three parameters be the same as the ones you use when adding the listener.

Mozilla does not support Internet Explorer's method of converting <script> tags into event handlers, which extends <script> with for and event attributes (see Table 5). It also does not support the attachEvent and detachEvent methods. Instead, you should use the addEventListener and removeEventListener methods. Internet Explorer does not support the W3C events specification. However, IE can be remediated to support addEventListener and removeEventListener.

Rich text editing

While Mozilla prides itself with being the most W3C web standards compliant browser, it does support nonstandard functionality, such as innerHTML and rich text editing, if no W3C equivalent exists.

Mozilla 1.3 introduced an implementation of Internet Explorer's designMode feature, which turns an HTML document into a rich text editor field. Once turned into the editor, commands can run on the document through the execCommand command. Mozilla also supports Internet Explorer's contentEditable attribute for making any widget editable. Alternatively, you could use an iframe to add a rich text editor.

Rich text differences

Mozilla supports the W3C standard of accessing iframe's document object through IFrameElmRef.contentDocument, while Internet Explorer requires you to access it through document.frames["IframeName"] and then access the resulting document:

<script>
function getIFrameDocument(aID) {
  var rv = null;

  // if contentDocument exists, W3C compliant (Mozilla)
  if (document.getElementById(aID).contentDocument){
    rv = document.getElementById(aID).contentDocument;
  } else {
    // IE
    rv = document.frames[aID].document;
  }
  return rv;
}
</script>

Another difference between Mozilla and Internet Explorer is the HTML that the rich text editor creates. Mozilla defaults to using CSS for the generated markup. However, Mozilla allows you to toggle between HTML and CSS mode using the useCSS execCommand and toggling it between true and false. Internet Explorer always uses HTML markup.

Mozilla (CSS):
  <span style="color: blue;">Big Blue</span>

Mozilla (HTML):
  <font color="blue">Big Blue</font>

Internet Explorer:
  <FONT color="blue">Big Blue</FONT>

Below is a list of commands that execCommand in Mozilla supports:

For more information, visit Rich-Text Editing in Mozilla.

XML differences

Mozilla has strong support for XML and XML-related technologies, such as XSLT and Web services. It also supports some non-standard Internet Explorer extensions, such as XMLHttpRequest.

How to handle XML

As with standard HTML, Mozilla supports the W3C XML DOM specification, which allows you to manipulate almost any aspect of an XML document. Differences between Internet Explorer's XML DOM and Mozilla are usually caused by Internet Explorer's nonstandard behaviors. Probably the most common difference is how they handle white space text nodes. Often when XML generates, it contains white spaces between XML nodes. Internet Explorer, when using Node.childNodes, will not contain these white space nodes. In Mozilla, those nodes will be in the array.

XML:
  <?xml version="1.0"?>
  <myXMLdoc xmlns:myns="http://myfoo.com">
    <myns:foo>bar</myns:foo>
  </myXMLdoc>

JavaScript:
  var myXMLDoc = getXMLDocument().documentElement;
  alert(myXMLDoc.childNodes.length);

The first line of JavaScript loads the XML document and accesses the root element (myXMLDoc) by retrieving the documentElement. The second line simply alerts the number of child nodes. Per the W3C specification, the white spaces and new lines merge into one text node if they follow each other. For Mozilla, the myXMLdoc node has three children: a text node containing a new line and two spaces; the myns:foo node; and another text node with a new line. Internet Explorer, however, does not abide by this and will return "1" for the above code, namely only the myns:foo node. Therefore, to walk the child nodes and disregard text nodes, you must distinguish such nodes.

As mentioned earlier, every node has a nodeType property representing the node type. For example, an element node has type 1, while a document node has type 9. To disregard text nodes, you must check for types 3 (text node) and 8 (comment node).

XML:
  <?xml version="1.0"?>
  <myXMLdoc xmlns:myns="http://myfoo.com">
    <myns:foo>bar</myns:foo>
  </myXMLdoc>

JavaScript:
  var myXMLDoc = getXMLDocument().documentElement;
  var myChildren = myXMLDoc.childNodes;

  for (var run = 0; run < myChildren.length; run++){
    if ( (myChildren[run].nodeType != 3) &&
          myChildren[run].nodeType != 8) ){
      // not a text or comment node
    };
  };

See Whitespace in the DOM for more detailed discussion and a possible solution.

XML data islands

Internet Explorer has a nonstandard feature called XML data islands, which allow you to embed XML inside an HTML document using the nonstandard HTML tag <xml>. Mozilla does not support XML data islands and handles them as unknown HTML tags. You can achieve the same functionality using XHTML; however, because Internet Explorer's support for XHTML is weak, this is usually not an option.

IE XML data island:

<xml id="xmldataisland">
  <foo>bar</foo>
</xml>

One cross-browser solution is to use DOM parsers, which parse a string that contains a serialized XML document and generates the document for the parsed XML. Mozilla uses the DOMParser object, which takes the serialized string and creates an XML document out of it. In Internet Explorer, you can achieve the same functionality using ActiveX. The object created using new ActiveXObject("Microsoft.XMLDOM") has a loadXML method that can take in a string and generate a document from it. The following code shows you how:

var xmlString = "<xml id=\"xmldataisland\"><foo>bar</foo></xml>";
var myDocument;

if (window.DOMParser) {
  // This browser appears to support DOMParser
  var parser = new DOMParser();
  myDocument = parser.parseFromString(xmlString, "text/xml");
} else if (window.ActiveXObject){
  // Internet Explorer, create a new XML document using ActiveX
  // and use loadXML as a DOM parser.
  myDocument = new ActiveXObject("Microsoft.XMLDOM");
  myDocument.async = false;

  myDocument.loadXML(xmlString);
} else {
  // Not supported.
}

See Using XML Data Islands in Mozilla for an alternative approach.

XMLHttpRequest

Internet Explorer allows you to send and retrieve XML files using MSXML's XMLHTTP class, which is instantiated through ActiveX using new ActiveXObject("Msxml2.XMLHTTP") or new ActiveXObject("Microsoft.XMLHTTP"). Since there is no standard method of doing this, Mozilla provides the same functionality in the global JavaScript XMLHttpRequest object. Since version 7 IE also supports the "native" XMLHttpRequest object.

After instantiating the object using new XMLHttpRequest(), you can use the open method to specify what type of request (GET or POST) you use, which file you load, and if it is asynchronous or not. If the call is asynchronous, then give the onload member a function reference, which is called once the request has completed.

Synchronous request:

var myXMLHTTPRequest = new XMLHttpRequest();
myXMLHTTPRequest.open("GET", "data.xml", false);

myXMLHTTPRequest.send(null);

var myXMLDocument = myXMLHTTPRequest.responseXML;

Asynchronous request:

var myXMLHTTPRequest;

function xmlLoaded() {
  var myXMLDocument = myXMLHTTPRequest.responseXML;
}

function loadXML(){
  myXMLHTTPRequest = new XMLHttpRequest();
  myXMLHTTPRequest.open("GET", "data.xml", true);
  myXMLHTTPRequest.onload = xmlLoaded;
  myXMLHTTPRequest.send(null);
}

Table 7 features a list of available methods and properties for Mozilla's XMLHttpRequest.

XSLT differences

Mozilla supports XSL Transformations (XSLT) 1.0. It also allows JavaScript to perform XSLT transformations and allows running XPath on a document.

Mozilla requires that you send the XML and XSLT files with an XML mimetype (text/xml or application/xml). This is the most common reason why XSLT won't run in Mozilla but will in Internet Explorer. Mozilla is strict in that way.

Internet Explorer 5.0 and 5.5 supported XSLT's working draft, which is substantially different than the final 1.0 recommendation. The easiest way to distinguish what version an XSLT file was written against is to look at the namespace. The namespace for the 1.0 recommendation is http://www.w3.org/1999/XSL/Transform, while the working draft's namespace is http://www.w3.org/TR/WD-xsl. Internet Explorer 6 supports the working draft for backwards compatibility, but Mozilla does not support the working draft, only the final recommendation.

If XSLT requires you to distinguish the browser, you can query the "xsl:vendor" system property. Mozilla's XSLT engine will report itself as "Transformiix" and Internet Explorer will return "Microsoft".

<xsl:if test="system-property('xsl:vendor') = 'Transformiix'">
  <!-- Mozilla specific markup -->
</xsl:if>
<xsl:if test="system-property('xsl:vendor') = 'Microsoft'">
  <!-- Internet Explorer specific markup -->
</xsl:if>

Mozilla also provides JavaScript interfaces for XSLT, allowing a Web site to complete XSLT transformations in memory. You can do this using the global XSLTProcessor JavaScript object. XSLTProcessor requires you to load the XML and XSLT files, because it needs their DOM documents. The XSLT document, imported by the XSLTProcessor, allows you to manipulate XSLT parameters.

XSLTProcessor can generate a standalone document using transformToDocument(), or it can create a document fragment using transformToFragment(), which you can easily append into another DOM document. Below is an example:

var xslStylesheet;
var xsltProcessor = new XSLTProcessor();

// load the xslt file, example1.xsl
var myXMLHTTPRequest = new XMLHttpRequest();
myXMLHTTPRequest.open("GET", "example1.xsl", false);
myXMLHTTPRequest.send(null);

// get the XML document and import it
xslStylesheet = myXMLHTTPRequest.responseXML;

xsltProcessor.importStylesheet(xslStylesheet);

// load the xml file, example1.xml
myXMLHTTPRequest = new XMLHttpRequest();
myXMLHTTPRequest.open("GET", "example1.xml", false);
myXMLHTTPRequest.send(null);

var xmlSource = myXMLHTTPRequest.responseXML;

var resultDocument = xsltProcessor.transformToDocument(xmlSource);

After creating an XSLTProcessor, you load the XSLT file using XMLHttpRequest. The XMLHttpRequest's responseXML member contains the XML document of the XSLT file, which is passed to importStylesheet. You then use the XMLHttpRequest again to load the source XML document that must be transformed; that document is then passed to the transformToDocument method of XSLTProcessor. Table 8 features a list of XSLTProcessor methods.