In many ways, the Cascading Style Sheets (CSS) specification represents a unique development in the history of the World Wide Web. In its inherent ability to allow richly styled structural documents, CSS is both a step forward and a step backward -- but it's a good step backward, and a needed one. To see what is meant by this, it is first necessary to understand how the Web got to the point of desperately needing something like CSS, and how CSS makes the web a better place for both page authors and web surfers.
Back in the dimly remembered early years of the Web (1990 -1993), HTML was a fairly lean little language. It was almost entirely composed of structural elements that were useful for describing things like paragraphs, hyperlinks, lists, and headings. It had nothing even remotely approaching tables, frames, or the complex markup we assume is a necessary part of creating web pages. The general idea was that HTML would be a structural markup language, used to describe the various parts of a document. There was very little said about how these parts should be displayed. The language wasn't concerned with appearance. It was just a clean little markup scheme.
Then came Mosaic.
Suddenly, the power of the World Wide Web was obvious to almost anyone who spent more than ten minutes playing with it. Jumping from one document to another was no harder than pointing the mouse cursor at a specially colored bit of text, or even an image, and clicking the mouse button. Even better, text and images could be displayed together, and all you needed to create a page was a plain text editor. It was free, it was open, and it was cool.
Web sites began to spring up everywhere. There were personal journals, university sites, corporate sites, and more. As number of sites increased, so did the demand for new HTML tags that would allow one effect or another. Authors started demanding that they be able to make text boldfaced, or italicized.
At the time, HTML wasn't equipped to handle these sorts of desires. You could declare a bit of text to be emphasized, but that wasn't necessarily the same as being italicized -- it could be boldfaced instead, or even normal text with a different color, depending on the user's browser and their preferences. There was nothing to ensure that what the author created was what the reader would see.
As a result of these pressures, markup elements like <B> and <I> started to creep into the language. Suddenly, a structural language started to become presentational.
Years later, we have inherited the flaws inherent in this process. Large parts of HTML 3.2 and HTML 4.0, for example, are devoted to presentational considerations. The ability to color and size text through the FONT element, to apply background colors and images to documents and tables, to space and pad the contents of table cells, and to make text blink on and off are all the legacy of the original cries for "more control!"
If you want to know why this is a bad thing, all it takes is a quick glance at any corporate web site's page markup. The sheer amount of markup in comparison to actual useful information is astonishing. Even worse, for most sites, the markup is almost entirely made up of tables and FONT tags, none of which conveys any real semantic meaning to what's being presented. From a structural standpoint, these pages are little better than random strings of letters.
For example, let's assume that for page titles, an author is using FONT tags instead of heading tags like H1, like this:
<FONT SIZE="+3" FACE="Helvetica" COLOR="red">Page Title</FONT>
Structurally speaking, the FONT tag has no meaning. This makes the document far less useful. What good is a FONT tag to a speech-synthesis browser, for example? If an author uses heading tags instead of FONT tags, the speaking browser can use a certain speaking style to read the text. With the FONT tag, the browser has no way to know that the text is any different from other text.
Why do authors run roughshod over structure and meaning like this? Because they want readers to see the page as they designed it. To use structural HTML markup is to give up a lot of control over a page's appearance, and it certainly doesn't allow for the kind of densely packed page designs that have become so popular over the years.
So what's wrong with this? Consider the following:
Unstructured pages make content indexing inordinately difficult. A truly powerful search engine would allow users to search just page titles, or only section headings within pages, or only paragraph text, or perhaps only those paragraphs that are marked as being important. In order to do this, however, the page contents must be contained within some sort of structural markup -- exactly the sort of markup most pages lack.
A lack of structure reduces accessibility. Imagine that you are blind, and rely on a speech-synthesis browser to browse the Web. Which would you prefer: a structured page that lets your browser read only section headings so you can choose which section you'd like to hear more about; or a page so lacking in structure that your browser is forced to read the entire thing with no indication of what's a heading, what's a paragraph, and what's important?
Advanced page presentation is only possible with some sort of document structure. Imagine a page in which only the section headings are shown, with an arrow next to each. The user can decide which section heading applies to him and click on it, thus revealing the text of that section.
Structured markup is easier to maintain. How many times have you spent long minutes hunting through someone else's HTML (or even your own) in search of the one little error that is messing up your page in one browser or another? How much time have you spent writing nested tables and FONT tags, just to get a sidebar with white hyperlinks in it? How many line-break tags have you inserted trying to get exactly the right separation between a title and the following text? By using structural markup, you can clean up your code and make it easier to find what you're looking for.
Granted, a fully structured document is a little plain. Due to that one single fact, a hundred arguments in favor of structural markup wouldn't sway a marketing department away from the kind of HTML so prevalent at the end of the twentieth century. What was needed was a way to combine structural markup with attractive page presentation.
This concept is nothing new. There have been many style sheet technologies proposed and created over the last few decades. These were intended for use in various industries and in conjunction with a variety of structural markup languages. The concept had been tested, used, and generally found to be a benefit to any environment where structure had to be presented. However, no style sheet solution was immediately available for use with HTML. Something had to be done to correct this problem.
Second, all of the inline elements in a given line are alignedaccording to their values for vertical-align. Bydefault, this will cause all text in the line to be aligned alongtheir baselines, but of course different
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Almost as simple is this: the sum of the horizontal components of anonfloated block-level element box always equals thewidth of the parent. Take two paragraphswithin a DIV, for example, whose margins have beenset to be 1em. The content width (in other words,the value of width) of the paragraph, plus itsleft and right padding, borders, and margins, always add up to thewidth of the DIV 'scontent, as illustrated in Figure 8-7.styles from the paragraph, so they may as well be contained within tags such as these. Therefore, the only reason we create line-height rules for block-level elements is so that we don't have to explicitly declare a line-height for all its inline elements, fictional or otherwise.
The fictional LINE element actually clarifies the behavior that results from setting line-height on