What Is Css-www.277.cc

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SEO Cascading Style Sheets (CSS) is a style sheet language used to describe the presentation (that is, the look and formatting) of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can be applied to any kind of XML document, including SVG and XUL. CSS is designed primarily to enable the separation of document content (written in HTML or a similar markup language) from document presentation, including elements such as the colors, fonts, and layout. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple pages to share formatting, and reduce complexity and repetition in the structural content (such as by allowing for table less web design). CSS can also allow the same markup page to be presented in different styles for different rendering methods, such as on-screen, in print, by voice (when read out by a speech-based browser or screen reader) and on Braille-based, tactile devices. While the author of a document typically links that document to a CSS style sheet, readers can use a different style sheet, perhaps one on their own computer, to override the one the author has specified. CSS specifies a priority scheme to determine which style rules apply if more than one rule matches against a particular element. In this so-called cascade, priorities or weights are calculated and assigned to rules, so that the results are predictable. Syntax CSS has a simple syntax, and uses a number of English keywords to specify the names of various style properties. A style sheet consists of a list of rules. Each rule or rule-set consists of one or more selectors and a declaration block. A declaration-block consists of a list of semicolon-separated declarations in braces. Each declaration itself consists of a property, a colon (:), a value, then a semi-colon (;) In CSS, selectors are used to declare which elements a style applies to, a kind of match expression. Selectors may apply to all elements of a specific type, or only those elements which match a certain attribute; elements may be matched depending on how they are placed relative to each other in the markup code, or on how they are nested within the document object model. In addition to these, a set of pseudo-classes can be used to define further behavior. Probably the best-known of these is: hover, which applies a style only when the user ‘points to’ the visible element, usually by holding the mouse cursor over it. It is appended to a selector as in a:hover or#elementid:hover. Other pseudo-classes and pseudo-elements are, for example: first-line, visited or: before. A special pseudo-class is: lang(c), "c". A pseudo-class selects entire elements, such as: link or: visited, whereas a pseudo-element makes a selection that may consist of partial elements, such as: first-line or: first-letter. Uses of CSS Prior to CSS, nearly all of the presentational attributes of HTML documents were contained within the HTML markup; all font colors, background styles, element alignments, borders and sizes had to be explicitly described, often repeatedly, within the HTML. CSS allows authors to move much of that information to a separate style sheet resulting in considerably simpler HTML markup. Headings (h1 elements), sub-headings (h2), sub-sub-headings (h3), etc., are defined structurally using HTML. In print and on the screen, choice offont, size, color and emphasis for these elements are presentational. Prior to CSS, document authors who wanted to assign such typographic characteristics to, say, all h2 headings had to use the HTML font and other presentational elements for each occurrence of that heading type. The additional presentational markup in the HTML made documents more complex, and generally more difficult to maintain. In CSS, presentation is separated from structure. In print, CSS can define color, font, text alignment, size, borders, spacing, layout and many other typographic characteristics. It can do so independently for on-screen and printed views. CSS also defines non-visual styles such as the speed and emphasis with which text is read out by aural text readers. The W3C now considers the advantages of CSS for defining all aspects of the presentation of HTML pages to be superior to other methods. It has therefore deprecated the use of the entire original presentational HTML markup. About the Author: 相关的主题文章: