Saturday 18th of November 2017 04:52:05 PM

CSS Style Guide negativity will depend on the width of the parent element. Thus:

P {margin: -10%;}

Figure 7-20 illustrates the consequences of such a rule, where the amount by which paragraphs overlap each other and spill beyond the browser window is entirely dependent on the width of the window itself -- and the wider the window, the worse the situation becomes.

Figure 7-20

Figure 7-20. The dangers of document-wide negative-margin rules

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!

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." />
« XHTML Section Index | XHTML Benefits »

with a little bit less typing. In general, once you're trying to set margins for more than one side, it's almost easier to simply use margin. From the standpoint of your document's display, however, it doesn't really matter which approach you use, so feel free to choose whichever is easier for you.

7.3.5. Collapsing Margins

bullet, unless you prevent this from happening:

UL {list-style-image: url(ohio.gif); list-style-type: square;}UL UL {list-style-image: none;}

Since the nested list inherits the item typesquare but has been set to use no image for itsbullets, squares are used for the bullets in the nested list, asshown in Figure 7-84.

Figure 7-84

Figure 7-84. Switching off image bullets in sublists

H1 element. In
Figure 7-6, thisis represented using dashed lines which are included for illustrativepurposes. These lines would not actually appear in a web browser.

margin can accept any length measure, whether inpixels, inches, millimeters, or ems. However, the default value formargin is effectively 0 (zero),which means that if you don't declare a value, then by default,there won't be a margin.