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 tableless 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 stylesheet, readers can use a different stylesheet, 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.
The CSS specifications are maintained by the World Wide Web Consortium (W3C). Internet media type (MIME type) text/css is registered for use with CSS by RFC 2318 (March 1998).
Advantages
[edit]Flexibility
By combining CSS with the functionality of a Content Management System, a considerable amount of flexibility can be programmed into content submission forms. This allows a contributor, who may not be familiar or able to understand or edit CSS or HTML code to select the layout of an article or other page they are submitting on-the-fly, in the same form. For instance, a contributor, editor or author of an article or page might be able to select the number of columns and whether or not the page or article will carry an image. This information is then passed to the Content Management System, and the program logic will evaluate the information and determine, based on a certain number of combinations, how to apply classes and IDs to the HTML elements, therefore styling and positioning them according to the pre-defined CSS for that particular layout type. When working with large-scale, complex sites, with many contributors such as news and informational sites, this advantage weighs heavily on the feasibility and maintenance of the project.
[edit]Separation of Content from Presentation
CSS facilitates publication of content in multiple presentation formats based on nominal parameters. Nominal parameters include explicit user preferences, different web browsers, the type of device being used to view the content (a desktop computer or mobile Internet device), the geographic location of the user and many other variables.
[edit]Site-wide consistency
Main articles: Separation of presentation and content and Style sheet (web development)
When CSS is used effectively, in terms of inheritance and "cascading," a global stylesheet can be used to affect and style elements site-wide. If the situation arises that the styling of the elements should need to be changed or adjusted, these changes can be made easily, simply by editing a few rules in the global stylesheet. Before CSS, this sort of maintenance was more difficult, expensive and time-consuming.
[edit]Bandwidth
A stylesheet will usually be stored in the browser cache, and can therefore be used on multiple pages without being reloaded, increasing download speeds and reducing data transfer over a network.
[edit]Page reformatting
Main article: Progressive enhancement
With a simple change of one line, a different stylesheet can be used for the same page. This has advantages for accessibility, as well as providing the ability to tailor a page or site to different target devices. Furthermore, devices not able to understand the styling will still display the content.
Limitations
This article's Criticism or Controversy section(s) may mean the article does not present a neutral point of view of the subject. It may be better to integrate the material in such sections into the article as a whole.
This article may contain original research or unverified claims. Please improve the article by adding references. See the talk pagefor details. (March 2009)
Some noted disadvantages of using "pure" CSS include:
Inconsistent browser support
Different browsers will render CSS layout differently as a result of browser bugs or lack of support for CSS features. For example Microsoft Internet Explorer, whose older versions, such as IE 6.0, implemented many CSS 2.0 properties in its own, incompatible way, misinterpreted a significant number of important properties, such as width, height, and float.[15]Numerous so-called CSS "hacks" must be implemented to achieve consistent layout among the most popular or commonly used browsers. Pixel precise layouts can sometimes be impossible to achieve across browsers.
Selectors are unable to ascend
CSS offers no way to select a parent or ancestor of element that satisfies certain criteria. A more advanced selector scheme (such as XPath) would enable more sophisticated stylesheets. However, the major reasons for the CSS Working Group rejecting proposals for parent selectors are related to browser performance and incremental rendering issues.
One block declaration cannot explicitly inherit from another
Inheritance of styles is performed by the browser based on the containment hierarchy of DOM elements and the specificity of the rule selectors, as suggested by the section 6.4.1 of the CSS2 specification.[16] Only the user of the blocks can refer to them by including class names into the class attribute of a DOM element.
Vertical control limitations
While horizontal placement of elements is generally easy to control, vertical placement is frequently unintuitive, convoluted, or impossible. Simple tasks, such as centering an element vertically or getting a footer to be placed no higher than bottom of viewport, either require complicated and unintuitive style rules, or simple but widely unsupported rules.[clarification needed]
Absence of expressions
There is currently no ability to specify property values as simple expressions (such as margin-left: 10% - 3em + 4px;). This is useful in a variety of cases, such as calculating the size of columns subject to a constraint on the sum of all columns. However, a working draft with a calc() value to address this limitation has been published by the CSS WG.[17] Internet Explorer versions 5 to 7 support a proprietary expression() statement,[18] with similar functionality. This proprietary expression() statement is no longer supported from Internet Explorer 8 onwards, except in compatibility modes. This decision was taken for "standards compliance, browser performance, and security reasons".[18]
Lack of orthogonality
Multiple properties often end up doing the same job. For instance, position, display and float specify the placement model, and most of the time they cannot be combined meaningfully. A display: table-cell element cannot be floated or given position: relative, and an element with float: left should not react to changes of display. In addition, some properties are not defined in a flexible way that avoids creation of new properties. For example, you should use the "border-spacing" property on table element instead of the "margin-*" property on table cell elements. This is because according to the CSS specification, internal table elements do not have margins.
Margin collapsing
Margin collapsing is, while well-documented and useful, also complicated and is frequently not expected by authors, and no simple side-effect-free way is available to control it.
Float containment
CSS does not explicitly offer any property that would force an element to contain floats. Multiple properties offer this functionality as a side effect, but none of them are completely appropriate in all situations. As there will be an overflow when the elements, which is contained in a container, use float property. Generally, either "position: relative" or "overflow: hidden"[19] solves this. Floats will be different according to the web browser size and resolution, but positions can not.
Lack of multiple backgrounds per element
Highly graphical designs require several background images for every element, and CSS can support only one. Therefore, developers have to choose between adding redundant wrappers around document elements, or dropping the visual effect. This is partially addressed in the working draft of the CSS3 backgrounds module,[20] which is already supported in Safari and Konqueror.
Control of Element Shapes
CSS currently only offers rectangular shapes. Rounded corners or other shapes may require non-semantic markup. However, this is addressed in the working draft of the CSS3 backgrounds module.[21]
Lack of Variables
CSS contains no variables. This makes it necessary to use error-prone "replace-all" techniques to change fundamental constants, such as the color scheme or various heights and widths. Server-side generation of CSS scripts, using for example PHP, can help to mitigate this problem.
Lack of column declaration
While possible in current CSS, layouts with multiple columns can be complex to implement. With the current CSS, the process is often done using floating elements which are often rendered differently by different browsers, different computer screen shapes, and different screen ratios set on standard monitors.
Cannot explicitly declare new scope independently of position
Scoping rules for properties such as z-index look for the closest parent element with a position:absolute or position:relative attribute. This odd coupling has two undesired effects: 1) it is impossible to avoid declaring a new scope when one is forced to adjust an element's position, preventing one from using the desired scope of a parent element and 2) users are often not aware that they must declare position:relative or position:absolute on any element they want to act as "the new scope". Additionally, a bug in the Firefox browser prevents one from declaring table elements as a new css scope using position:relative (one can technically do so, but numerous graphical glitches result).
Poor Layout Controls for Flexible Layouts
While new additions to CSS3 provide a stronger, more robust layout feature-set, CSS is still very much rooted as a styling language, not a layout language. This problem has also meant that creating fluid layouts is still very much done by hand-coding CSS, and make the development of a standards-based WYSIWYG editor more difficult than expected.
Why to use CSS, its advantages
CSS stands for Cascading Style Sheet. CSS is used to improve the presentation of the content within HTML tags. Using CSS we can define different ways of content display based on the media.
WE will discuss some advantages of using CSS
Less code
As we define the property with value at a common style class and use the same class at different locations, so we use less code. Less code means less bandwidth consumption and easy to maintain the content.
Easy maintenance
As we define styles globally or at a common place so any changes became easy. For example in a site we have all the name of the products is displayed using a particular style property. Now by changing the style class at external style sheet for the product name we can change the style through out the site. We can keep more than one style sheet and use them based on requirement. With property inheritance methods maintaining of different styles of the same tag became easy.
Higher content to code ( tag ) ratio
We can achieve higher content to code ration in a page by using CSS as we can shift the style declarations to an external file. This is important for search engine point of view. We present less tags ( style tags ) and more content to the spiders for indexing.
First download of pages.
As browsers cache the style sheet page so the page loading became fast. The styles classes are not loaded from server each time different pages of same site using same CSS is used.
Flexibility in defining style.
The name cascading indicates that we can use more than one style and the priority is given to local styles first. We can override the global style declared and locally assign the style to the tag.
Type Of Selectors
Class selectors
Used to define styles that can be used without redefining plain HTML tags.
ID selectors
Used to define styles relating to objects with a unique ID (most often layers)
The general syntax for a Class selector is:
.ClassSelector {Property:Value;}
For example:
This is a bold tag carrying the headline class
This is an italics tag carrying the headline class
Click here to open a window that shows the result of the above example.
Class selectors are used when you want to define a style that does not redefine an HTML tag entirely.
When referring to a Class selector you simply add the class to an HTML tag like in the above example (class="headline").
________________________________________
The general syntax for an ID selector is:
#IDSelector {Property:Value;}
For example:
| THIS IS LAYER 1 POSITIONED AT 100,100 |
| THIS IS LAYER 2 POSITIONED AT 140,140 |
Click here to open a window that shows the result of the above example.
ID selectors are used when you want to define a style relating to an object with a unique ID.
This selector is most widely used with layers (as in the above example), since layers are always defined with a unique ID.
SPAN and DIV as carriers
Two tags are particularly useful in combination with class selectors: and
.
Both are "dummy" tags that don't do anything in themselves. Therefore, they are excellent for carrying CSS styles.
is an "inline-tag" in HTML, meaning that no line breaks are inserted before or after the use of it.
Both are "dummy" tags that don't do anything in themselves. Therefore, they are excellent for carrying CSS styles.
is an "inline-tag" in HTML, meaning that no line breaks are inserted before or after the use of it.
is a "block tag", meaning that line breaks are automatically inserted to distance the block from the surrounding content (like tags).
or
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