Tuesday 07th of February 2012 05:27:25 AM

CSS Style Guide

XHTML: Guidelines

All developers are familiar with HTML, the web’s original markup language. But the W3C currently recommends using XHTML instead. This hybrid language looks and works much like HTML but is based on XML, the web’s “super” markup language.

The Library has standardized on XHTML 1.0 Transitional, a version of XHTML that works well in both old and new browsers, and that accommodates the needs of older browsers (such as Netscape Navigator 4 and Internet Explorer 4) that are used by a significant portion of the NYPL audience.

Rules of XHTML

Converting from traditional HTML to XHTML 1.0 Transitional is easy, as long as you work carefully and observe the following rules:

1. Open with the proper DOCTYPE & Namespace

XHTML documents must begin with tags that tell the browser how to interpret them. All library web pages must begin with the following DOCTYPE declaration:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd ">

The declaration above should be typed (or cut and pasted) into the very top of every XHTML document, before any other code or markup. View Source on this page to familiarize yourself with the proper placement of this DOCTYPE.

The XHTML 1.0 Transitional DOCTYPE must be followed by this XHTML namespace declaration:

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

Once again, View Source to familiarize yourself with the proper placement of the namespace declaration.

Note: many XHTML pages begin with an optional XML prologue (<?xml>) that precedes the DOCTYPE and namespace declarations. Unfortunately, this XML prologue causes problems in many browsers and must be omitted from NYPL web pages.

One of the main purposes of the prologue is to specify character encoding within your document. If you’re working on an international site and your page will include non–ASCII characters, you can probably get by with a simple meta tag such as:

<meta http-equiv="Content-Type" content="text/html;charset=UTF-8" />
2. Write all tags in lowercase

Unlike HTML, XML is case–sensitive. All XHTML tags and attributes must be typed in lowercase, or your document will not validate. (Validation ensures that your pages are error-free. See the section on validation if you are unfamiliar with this subject.)

In order to "translate" an older document to XHTML, the following markup ...

<TITLE>New York Public Library</TITLE>

... would be recast thusly:

<title>New York Public Library</title>

Likewise, <P> becomes <p>, <BODY> becomes <body>, and so on.

3. Quote all attribute values

In HTML, you needn’t put quotation marks around attribute values. In XHTML, they must be quoted, e.g., height="55", not height=55.

4. Close all tags

In HTML, you have the option to open many tags such as <p> and <li> without closing them:

<p>This would be invalid XHTML.
<p>I forgot to close my Paragraph tags! elements exist almost on their own plane, yet they still have majorinfluence over the rest of the document.

Of course, when an element is floated, other content "flowsaround" it. This is familiar behavior with floated images, butthe same is true if you float a paragraph, for example. In Figure 7-64, we can see this effect (a margin has beenadded to make the situation more clear):

P.aside {float: left; width: 5em; margin: 1em;}
Figure 7-64

Figure 7-64. A floating paragraph

In XHTML, every tag that opens must close:

<p>This is valid XHTML.</p>
<p>I close my tags after opening them.</p>
5. Close “empty” tags, too

In XHTML, even “empty” tags such as <br> and <img> must close themselves by including a forward slash /> at the very end of the tag:

<br />
<img src="library.gif" />

Note the slash /> at the very end. Note also that a single blank space precedes the slash to avoid confusing older browsers that were released prior to the XHTML standard.

Important: To remain valid and accessible, the image tag in the second example would also have to include an “alt” attribute, and an optional “title” attribute wouldn’t hurt:

<img src="library.gif" alt="New York Public Library" title="A view of the main Library at 42nd Street and Fifth Avenue, Manhattan." />
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TIP

These min-max properties can be very useful in conjunction withloated elements as well. For example, youcan allow a floated element's width to be relative to the widthof its parent element (which is its containing block), while alsomaking sure that the float's width never goes below10em. The reverse approach is also possible:

This will set the float to be 40em wide, unlessthat would be more than 40% the width of the containing block, in

In a like manner, setting a negative value on the other sides will pull them beyond their normal limits: 

<P STYLE="margin: -2em; font-weight: bold;">...

As Figure 7-19 makes abundantly clear, the paragraph has spilled beyond the edges of the browser window and has not only pulled up far enough to overlap the end of the previous paragraph, but has also pulled the following paragraph up to overlap its last line.<!-- ...or, to put it another way... --> <P> bold <SPAN> bold <STRONG> regular <B> regular <STRONG> regular </STRONG></B></STRONG></SPAN>. </P>

Ignoring the fact that this would be entirely counterintuitive, what we see in Figure 5-16 is that the main paragraph text has a weight of 900 and the SPAN aweight of 700. When the STRONG text isbug in Navigator 4.x, which interprets it asblack.

Example

Sets animage to be the background pattern. Depending on the value ofbackground-repeat, the background image may tileindefinitely, or only along one axis, or not at all, and the startingposition of the tiling is dependent on the value ofbackground-position.