Tags and Attributes (HTML & XHTML: The Definitive Guide, 4th Edition)

3.3.1. The Syntax of a Tag

Every tag consists of a tag name, sometimes followed by an optional list of tag attributes, all placed between opening and closing brackets (< and >). The simplest tag is nothing more than a name appropriately enclosed in brackets, such as <head> and . More complicated tags contain one or more attributes, which specify or modify the behavior of the tag.

According to the HTML standard, tag and attribute names are not case-sensitive. There's no difference in effect between <head>, <Head>, <HEAD>, or even <HeaD>; they are all equivalent. With XHTML, case is important: all current standard tag and attribute names are in lowercase.

For both HTML and XHTML, the values that you assign to a particular attribute may be case-sensitive, depending on your browser and server. In particular, file location and name references -- or uniform resource locators (URLs) -- are case-sensitive. Section 6.2, "Referencing Documents: The URL"

Tag attributes, if any, belong after the tag name, each separated by one or more tab, space, or return characters. Their order of appearance is not important.

A tag attribute's value, if any, follows an equal sign (=) after the attribute name. You may include spaces around the equal sign, so that width=6, width = 6, width =6, and width= 6 all mean the same. For readability, however, we prefer not to include spaces. That way, it's easier to pick out an attribute/value pair from a crowd of pairs in a lengthy tag.

With HTML, if an attribute's value is a single word or number (no spaces), you may simply add it after the equal sign. All other values should be enclosed in single or double quotation marks, especially those values that contain several words separated by spaces. With XHTML, all attribute values must be enclosed in double-quotes. The length of the value is limited to 1024 characters.

Most browsers are tolerant of how tags are punctuated and broken across lines. Nonetheless, avoid breaking tags across lines in your source document whenever possible. This rule promotes readability and reduces potential errors in your HTML documents.

3.3.3. Starting and Ending Tags

We alluded earlier to the fact that most tags have a beginning and an end and affect the portion of content between them. That enclosed segment may be large or small, from a single text character, syllable, or word, such as the italicized "simp" syllable in our barebones example, to the <html> tag that bounds the entire document. The starting component of any tag is the tag name and its attributes, if any. The corresponding ending tag is the tag name alone, preceded by a slash. Ending tags have no attributes.

3.3.4. Proper and Improper Nesting

Tags can be put inside the affected segment of another tag (nested) for multiple tag effects on a single segment of the document. For example, a portion of the following text is both bold and included as part of an anchor defined by the <a> tag:

<body>
This is some text in the body, with a
<a href="another_doc.html">link, a portion of which
is <b>set in bold</b></a>
</body>

According to the HTML and XHTML standards, you must end nested tags starting with the most recent one and work your way back out. For instance in the example, we end the bold tag () before ending the link tag (</a>) since we started in the reverse order: <a> tag first, then  tag. It's a good idea to follow that standard, even though most browsers don't absolutely insist you do so. You may get away with violating this nesting rule for one browser, sometimes even with all current browsers. But eventually a new browser version won't allow the violation and you'll be hard pressed to straighten out your source HTML document. And, be aware that the XHTML standard explicitly forbids improper nesting.

3.3.5. Tags Without Ends

According to the HTML standard, a few tags do not have an ending tag. In fact, the standard forbids use of an end tag for these special ones, although most browsers are lenient and ignore the errant end tag. For example, the   tag causes a line break; it has no effect otherwise on the subsequent portion of the document and, hence, does not need an ending tag.

The HTML tags that do not have a corresponding end tags are:

<area>
<base>
<basefont>
<br>
<col>
<frame>
<hr>
<img>
<input>
<isindex>
<link>
<meta>
<param>

XHTML always requires end tags. Section 16.3.3, "Handling Empty Elements"

3.3.6. Omitting Tags

You often see documents in which the author seemingly has forgotten to include an ending tag in apparent violation of the HTML standard. Sometimes you even see a missing <body> tag. But your browser doesn't complain, and the document displays just fine. What gives? The HTML standard lets you omit certain tags or their endings for clarity and ease of preparation. The HTML standard writers didn't intend the language to be tedious.

For example, the  tag that defines the start of a paragraph has a corresponding end tag , but the  ending tag rarely is used. In fact, many HTML authors don't even know it exists! Section 4.1.2, "The Tag"

Rather, the HTML standard lets you omit a starting tag or ending tag whenever it can be unambiguously inferred by the surrounding context. Many browsers make good guesses when confronted with missing tags, leading the document author to assume that a valid omission was made.

We recommend that you most always add the ending tag. It'll make life easier for yourself as you transition to XHTML, as well as on the browser and anyone who might need to modify your document in the future.

3.3.7. Ignored or Redundant Tags

HTML browsers sometimes ignore tags. This usually happens with redundant tags whose effects merely cancel or substitute for themselves. The best example is a series of  tags, one after the other with no intervening content. Unlike the similar series of repeating return characters in a text-processing document, most browsers skip to a new line only once. The extra  tags are redundant and usually ignored by the browser.

In addition, most HTML browsers ignore any tag that they don't understand or that was incorrectly specified by the document author. Browsers habitually forge ahead and make some sense of a document, no matter how badly formed and error-ridden it may be. This isn't just a tactic to overcome errors; it's also an important strategy for extensibility. Imagine how much harder it would be to add new features to the language if the existing base of browsers choked on them.

The thing to watch out for with nonstandard tags that aren't supported by most browsers is their enclosed contents, if any. Browsers that recognize the new tag may process those contents differently than those that don't support the new tag. For example, Internet Explorer and Netscape Navigator now both support the <style> tag, whose contents serve to set the variety of display characteristics of your document. However, previous versions of the popular browsers, many of which are still in use by many people today, don't support styles. Hence, older browsers ignore the <style> tag and render its contents on the user's screen, effectively defeating the tag's purpose in addition to ruining the document's appearance. Section 8.1.2, "Document-Level Style Sheets"

3.3. Tags and Attributes