Web style sheets are a form of separation of presentation and content Separation of presentation and content is a common idiom, a design philosophy, and a methodology applied in the context of various publishing technology disciplines, including information retrieval, , web design, web development, word processing, desktop publishing, and model-driven development. It is a specific instance of the more general for web design Web design is the skill of creating presentations of content that is delivered to an end-user through the World Wide Web, by way of a Web browser or other Web-enabled software like Internet television clients, microblogging clients and RSS readers in which the markup A markup language is a set of annotations to text that describe how it is to be structured, laid out, or formatted. Markup languages might be manuscript form , or they might be markup codes used in computer typesetting and word-processing systems. The former are also commonly used to describe the required layout of papers, articles, standards, or (i.e., HTML HTML, an initialism for Hypertext Mark-up Language, is the predominant markup language for web pages. It provides a means to describe the structure of text-based information in a document—by denoting certain text as links, headings, paragraphs, lists, etc.—and to supplement that text with interactive forms, embedded images, and other objects or XHTML The Extensible Hypertext Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax) of a webpage A web page or webpage is a document or resource of information that is suitable for the World Wide Web and can be accessed through a web browser and displayed on a computer screen contains the page's semantic content and structure, but does not define its visual layout (style). Instead, the style is defined in an external stylesheet file using a language such as CSS Cascading Style Sheets 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 or XSL In computing, the Extensible Stylesheet Language , a family of transformation languages, allows one to describe how to format or transform files encoded in the XML standard. The XSL language itself uses valid XML syntax, with constructs such as:. This design approach is identified as a "separation" because it largely supersedes the antecedent methodology in which a page's markup defined both style and structure.
The philosophy underlying this methodology is a specific case of separation of concerns In computer science, separation of concerns is the process of separating a computer program into distinct features that overlap in functionality as little as possible. A concern is any piece of interest or focus in a program. Typically, concerns are synonymous with features or behaviors. Progress towards SoC is traditionally achieved through.
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Benefits
Separation of style and content has many benefits, but has only become practical in recent years due to improvements in popular web browsers A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content. Hyperlinks present in resources enable users to easily navigate their browsers to' CSS implementations.
Speed
Overall, users experience of a site utilising style sheets will generally be quicker than sites that don’t use the technology. ‘Overall’ as the first page will probably load more slowly – because the style sheet AND the content will need to be transferred. Subsequent pages will load faster because no style information will need to be downloaded – the CSS file will already be in the browser's cache.
Maintainability
Holding all the presentation styles in one file significantly reduces maintenance time and reduces the chance of human errors, thereby improving presentation consistency. For example, the font color associated with a type of text element may be specified — and therefore easily modified — throughout an entire website simply by changing one short string of characters in a single file. The alternate approach, using styles embedded in each individual page, would require a cumbersome, time consuming, and error-prone edit of every file.
Accessibility
Sites that use CSS with either XHTML or HTML are easier to tweak so that they appear extremely similar in different browsers (Internet Explorer Windows Internet Explorer , is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995. It has been the most widely used web browser since 1999, attaining a peak of about 95% usage share during 2002 and 2003 with IE5 and IE6. That percentage share has since, Mozilla Firefox Mozilla Firefox is a free and open source web browser descended from the Mozilla Application Suite and managed by Mozilla Corporation. Firefox had 22.51% of the recorded usage share of web browsers as of May 2009[update], making it the second most popular browser in terms of current use worldwide, after Internet Explorer. See how Firefox performs, Opera Opera is a web browser and internet suite developed by the Opera Software company. Opera handles common Internet-related tasks such as displaying web sites, sending and receiving e-mail messages, managing contacts, IRC online chatting, downloading files via BitTorrent, and reading web feeds. Opera is offered free of charge for personal computers, Safari Safari is a web browser developed by Apple Inc. First released as a public beta on January 7, 2003 on the company's Mac OS X operating system, it became Apple's default browser beginning with Mac OS X v10.3 "Panther". Apple has also made Safari the native browser for the iPhone OS. A version of Safari for the Microsoft Windows operating, etc.).
Sites using CSS "degrade gracefully Fault-tolerance or graceful degradation is the property that enables a system to continue operating properly in the event of the failure of (or one or more faults within) some of its components. If its operating quality decreases at all, the decrease is proportional to the severity of the failure, as compared to a naïvely-designed system in which" in browsers unable to display graphical content, such as Lynx, or those so very old that they cannot use CSS. Browsers ignore CSS that they do not understand, such as CSS 3 statements. This enables a wide variety of user agents A user agent is the client application used with a particular network protocol; the phrase is most commonly used in reference to those which access the World Wide Web. Other systems, such as Session Initiation Protocol , use the term user agent to refer to both end points of a phone call, server and client to be able to access the content of a site even if they cannot render the stylesheet or are not designed with graphical capability in mind. For example, a browser using a refreshable braille display for output could disregard layout information entirely, and the user would still have access to all page content.
Customization
If a page's layout information is all stored externally, a user can decide to disable the layout information entirely, leaving the site's bare content still in a readable form. Site authors may also offer multiple stylesheets, which can be used to completely change the appearance of the site without altering any of its content.
Most modern web browsers also allow the user to define their own stylesheet, which can include rules that override the author's layout rules. This allows users, for example, to bold every hyperlink on every page they visit.
Consistency
Because the semantic file contains only the meanings an author intends to convey, the styling of the various elements of the document's content is very consistent. For example, headings, emphasized text, lists and mathematical expressions all receive consistently applied style properties from the external stylesheet. Authors need not concern themselves with the style properties at the time of composition. These presentational details can be deferred until the moment of presentation.
Portability
The deferment of presentational details until the time of presentation means that a document can be easily re-purposed for an entirely different presentation medium with merely the application of a new stylesheet already prepared for the new medium and consistent with elemental or structural vocabulary of the semantic document. A carefully authored document for a web page can easily be printed to a hard-bound volume complete with headers and footers, page numbers and a generated table of contents simply by applying a new stylesheet.
Practical disadvantages today
Currently specifications (for example, XHTML, XSL, CSS) and software tools implementing these specification are only reaching the early stages of maturity. So there are some practical issues facing authors who seek to embrace this method of separating content and style.
Complex layouts
One of the practical problems is the lack of proper support for style languages in major browsers. Typical web page layouts call for some tabular presentation of the major parts of the page such as menu navigation columns and header bars, navigation tabs, and so on. However, deficient support for CSS and XSL in major browsers forces authors to code these tables within their content rather than applying a tabular style to the content from the accompanying stylesheet.
Narrow adoption without the parsing and generation tools
While the style specifications are quite mature and still maturing, the software tools have been slow to adapt. Most of the major web development tools still embrace a mixed presentation-content model. So authors and designers looking for GUI based tools for their work find it difficult to follow the semantic web method. In addition to GUI tools, shared repositories for generalized stylesheets would probably aid adoption of these methods.
See also
External Resources
- Simply JavaScript: The Three Layers of the Web
- CSS Zen Garden: A site which challenges designers to create new page layouts without touching the XHTML source. Includes dozens of layouts. CSS source can be viewed for every layout.
- Revealed - Our HTML and CSS Codes - cameraontheroad.com: Gives practical examples of CSS advantages from a site which has moved from embedded presentation styles to style sheets. Includes links to CSS resources.
- Brugbart: A site holding Tutorials and References to CSS based, and standard compliant layouts.
- SLIRK: A new css/javascript website layout approach.
Categories: Web development
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