Book HomeCascading Style Sheets: The Definitive GuideSearch this book Friday 28th of April 2017 06:22:09 AM

Preface

The subject of this book is, as you might have guessed by the cover, Cascading Style Sheets (CSS). There are two "levels" to CSS; these are referred to as CSS1 and CSS2. The difference between the two is that CSS2 is all of CSS1, plus a lot more. This book attempts to cover all of CSS1, and CSS positioning, which is a part of CSS2. The rest of CSS2 is excluded because, at the time of this writing, nobody had implemented most of it. Rather than cover a lot of theoretical territory, we chose to stick to what was currently usable.

If you are a web designer or document author interested in sophisticated page styling, improved accessibility, and saving time and effort, then this book is for you. All you really need before starting the book is a decent knowledge of HTML 4.0. The better you know HTML, of course, the better prepared you'll be. You will need to know very little else in order to follow this book.

It is important to remember something about web standards and books: the former are continually evolving, while the latter are frozen in time (until the next edition comes out, anyway). In the case of HTML and CSS, there are a great many changes afoot even as these words are being written. The recent formalization of XHTML 1.0 as a full W3C Recommendation, for example, is a major milestone in the evolution of the World Wide Web. There are likely to be even more levels to CSS, further extending the ability to style documents; major web browsers are approaching full CSS1 support, and robust CSS2 implementations can be seen lurking on the horizon. This is an exciting time to be a designer, and learning CSS now will give you a leg up on the future.

0.1. Typographical Conventions

The following typographical conventions are used in this book:

Constant width

is used to indicate code examples, HTML tags and CSS elements.

Constant width italic

is used for replaceables that appear in text.

Italic

is used to introduce new terms and to indicate URLs, filenames, and pathnames.

TIP

indicates a note or tip relating to the nearby text.

WARNING

indicates a warning.

If a vertically aligned element doesn't have abaseline -- that is, if it's animage, a form input, or another replacedelement -- then the bottom of the element is aligned with thebaseline of its parent, as Figure 4-31 shows:

IMG {vertical-align: baseline;}<P>The image found in this paragraph <IMG SRC="dot.gif" ALT="a dot"> has itsbottom edge aligned with the baseline of the paragraph.</P>


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By making the W3C the keeper of the XML standard, it ensures that no one vendor should be able to cause interoperability problems to occur between systems that use the open standard. This should be reassuring to most companies making an investment in this technology, by being vendor neutral, this solution proposes to keep even small companies out of reach of big companies choosing to change the standards on them. For example, if a big company chooses to change the platform at its whim, then most other companies relying on that platform suffer. By keeping all data in XML and using XML in communications protocols, companies can maximize the lifetime of their investment in their products and solutions.

XML is language independent

By being language independent, XML bypasses the requirement to have a standard binary encoding or storage format. Language independence also fosters immense interoperability amongst heterogeneous systems. It is also good for future compatilbilty. For example, if in the future a product needs to be changed in order to deal with a new computing paradigm or network protocol, by keeping XML flowing through the system, addition of a new layer to deal with this change is feasible.

DOM and SAX are open, language-independent set of interfaces

already halfway to an understanding of how it works. Most of what remain are the details of what happens when absolute positioning is invoked.

When an element is positioned absolutely, it is completely removed from the document flow. It is then positioned with respect to its containing block, and its edges are placed using the side-offset properties. The positioned element does not flow around the content of other elements, nor does their content flow around the positioned element. This implies that an absolutely positioned element mayelements of any kind, are ignored for the purposes of this value.Instead, a "default" text box is considered. This defaultbox is derived from the font-size of the parentelement. The bottom of the aligned element's inline box is thenaligned with the bottom of default text box. Thus, given thefollowing markup, we get a situation such as that shown in Figure 4-35:

IMG.tbot {vertical-align: text-bottom;}<P>Here: a <IMG SRC="tall.gif" ALIGN="middle"ALT="tall image"> tall image, and then a <IMG SRC="short.gif"