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syntax and uses a number of english keywords to specify the names of various style properties style sheet edit main article style sheet web development a style sheet consists of a list of rules each rule or rule set consists of one or more selectors and a declaration block selector edit css class redirects here for element classes that are part of the html content model upon which the css class selector syntax depends see class attribute html in css selectors declare which part of the markup a style applies to by matching tags and attributes in the markup itself selector types edit selectors may apply to the following all elements of a specific type e g the second level headers h2 elements specified by attribute in particular id an identifier unique within the document denoted in the selector language by a hash prefix e g id class an identifier that can annotate multiple elements in a document denoted by a dot prefix e g classname the phrase css class although sometimes used is a misnomer as element classes specified with the html class attribute is a markup feature that is distinct from browsers css subsystems and the related w3c whatwg standards work on document styles see rdf and microformats for the origins of the class system of the web content model elements depending on how they are placed relative to others in the document tree classes and ids are case sensitive start with letters and can include alphanumeric characters hyphens and underscores a class may apply to any number of instances of any element an id may only be applied to a single element pseudo classes edit pseudo element redirects here for pseudoelement symbols in chemistry see skeletal formula pseudoelement symbols pseudo classes are used in css selectors to permit formatting based on information that is not contained in the document tree one example of a widely used pseudo class is hover which identifies content 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 a pseudo class classifies document 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 7 note the distinction between the double colon notation used for pseudo elements and the single colon notation used for pseudo classes combinators edit multiple simple selectors may be joined using combinators to specify elements by location element type id class or any combination thereof 8 the order of the selectors is important for example div myclass color red applies to all elements of class myclass that are inside div elements whereas myclass div color red applies to all div elements that are inside elements of class myclass this is not to be confused with concatenated identifiers such as div myclass color red which applies to div elements of class myclass summary of selector syntax edit the following table provides a summary of selector syntax indicating usage and the version of css that introduced it 9 css selectors pattern matches first defined in css level e an element of type e 1 e link an e element that is the source anchor of a hyperlink whose target is either not yet visited link or already visited visited 1 e visited e active an e element during certain user actions 1 e hover 2 e focus e first line the first formatted line of an e element 1 e first letter the first formatted letter of an e element 1 c all elements with class c 1 myid the element with id myid 1 e warning an e element whose class is warning the document language specifies how class is determined 1 e myid an e element with id equal to myid 1 c myid the element with class c and id equal to myid 1 e f an f element descendant of an e element 1 any element 2 e foo an e element with a foo attribute 2 e foo bar an e element whose foo attribute value is exactly equal to bar 2 e foo bar an e element whose foo attribute value is a list of whitespace separated values one of which is exactly equal to bar 2 e foo en an e element whose foo attribute has a hyphen separated list of values beginning from the left with en 2 e first child an e element first child of its parent 2 e lang fr an element of type e in language fr the document language specifies how language is determined 2 e before generated content before an e element s content 2 e after generated content after an e element s content 2 e f an f element child of an e element 2 e f an f element immediately preceded by an e element 2 e foo bar an e element whose foo attribute value begins exactly with the string bar 3 e foo bar an e element whose foo attribute value ends exactly with the string bar 3 e foo bar an e element whose foo attribute value contains the substring bar 3 e root an e element root of the document 3 e nth child n an e element the n th child of its parent 3 e nth last child n an e element the n th child of its parent counting from the last one 3 e nth of type n an e element the n th sibling of its type 3 e nth last of type n an e element the n th sibling of its type counting from the last one 3 e last child an e element last child of its parent 3 e first of type an e element first sibling of its type 3 e last of type an e element last sibling of its type 3 e only child an e element only child of its parent 3 e only of type an e element only sibling of its type 3 e empty an e element that has no children including text nodes 3 e target an e element being the target of the referring uri 3 e enabled a user interface element e that is enabled 3 e disabled a user interface element e that is disabled 3 e checked a user interface element e that is checked for instance a radio button or checkbox 3 e not s an e element that does not match simple selector s 3 e f an f element preceded by an e element 3 e has s an e element that contains an element matching simple selector s 4 declaration block edit a declaration block consists of a pair of braces enclosing a semicolon separated list of declarations 10 declaration edit each declaration itself consists of a property a colon and a value optional white space may be around the declaration block declarations colons and semi colons for readability 11 properties edit properties are specified in the css standard each property has a set of possible values some properties can affect any type of element and others apply only to particular groups of elements 12 13 values edit values may be keywords such as center or inherit or numerical values such as 200px 200 pixels 50vw 50 percent of the viewport width or 80 80 percent of the parent element s width color values can be specified with keywords e g red hexadecimal values e g ff0000 also abbreviated as f00 rgb values on a 0 to 255 scale e g rgb 255 0 0 rgba values that specify both color and alpha transparency e g rgba 255 0 0 0 8 or hsl or hsla values e g hsl 0 100 50 hsl 0 100 50 0 8 14 non zero numeric values representing linear measures must include a length unit which is either an alphabetic code or abbreviation as in 200px or 50vw or a percentage sign as in 80 some units cm centimetre in inch mm millimetre pc pica and pt point are absolute which means that the rendered dimension does not depend upon the structure of the page others em em ex ex and px pixel clarification needed are relative which means that factors such as the font size of a parent element can affect the rendered measurement these eight units were a feature of css 1 15 and retained in all subsequent revisions the proposed css values and units module level 3 will if adopted as a w3c recommendation provide seven further length units ch q rem vh vmax vmin and vw 16 use edit before css nearly all presentational attributes of html documents were contained within the html markup that is all font colors background styles element alignments borders and sizes had to be explicitly described often repeatedly within the html css lets authors move much of that information to another file the style sheet resulting in considerably simpler html and additionally as more and more devices are able to access responsive web pages different screen sizes and layouts begin to appear customizing a website for each device size is costly and increasingly difficult the modular nature of css means that styles can be reused in different parts of a site or even across sites promoting consistency and efficiency for example headings h1 elements sub headings h2 sub sub headings h3 etc are defined structurally using html in print and on the screen choice of font size color and emphasis for these elements is presentational before css document authors who wanted to assign such typographic characteristics to say all h2 headings had to repeat html presentational markup for each occurrence of that heading type this made documents more complex larger and more error prone and difficult to maintain css allows the separation of presentation from structure css can define color font text alignment size borders spacing layout and many other typographic characteristics and can do so independently for on screen and printed views css also defines non visual styles such as reading speed and emphasis for aural text readers the w3c has now deprecated the use of all presentational html markup 17 for example under pre css html a heading element defined with red text would be written as h1 font color red chapter 1 font h1 using css the same element can be coded using style properties instead of html presentational attributes h1 style color red chapter 1 h1 the advantages of this may not be immediately clear but the power of css becomes more apparent when the style properties are placed in an internal style element or even better an external css file for example suppose the document contains the style element style h1 color red style all h1 elements in the document will then automatically become red without requiring any explicit code if the author later wanted to make h1 elements blue instead this could be done by changing the style element to style h1 color blue style rather than by laboriously going through the document and changing the color for each individual h1 element the styles can also be placed in an external css file as described below and loaded using syntax similar to link href path to file css rel stylesheet type text css this further decouples the styling from the html document and makes it possible to restyle multiple documents by simply editing a shared external css file sources edit css information can be provided from various sources these sources can be the web browser the user and the author the information from the author can be further classified into inline media type importance selector specificity rule order inheritance and property definition css style information can be in a separate document or it can be embedded into an html document multiple style sheets edit multiple style sheets can be imported different styles can be applied depending on the output device being used for example the screen version can be quite different from the printed version so authors can tailor the presentation appropriately for each medium cascading edit the style sheet with the highest priority controls the content display declarations not set in the highest priority source are passed on to a source of lower priority such as the user agent style the process is called cascading one of the goals of css is to allow users greater control over presentation someone who finds red italic headings difficult to read may apply a different style sheet depending on the browser and the website a user may choose from various style sheets provided by the designers or may remove all added styles and view the site using the browser s default styling or may override just the red italic heading style without altering other attributes browser extensions like stylish and stylus have been created to facilitate the management of such user style sheets in the case of large projects cascading can be used to determine which style has a higher priority when developers do integrate third party styles that have conflicting priorities and to further resolve those conflicts additionally cascading can help create themed designs which help designers fine tune aspects of a design without compromising the overall layout css priority scheme edit css priority scheme highest to lowest priority css source type description 1 importance the important annotation overwrites the previous priority types 2 inline a style applied to an html element via html style attribute 3 media type a property definition applies to all media types unless a media specific css is defined 4 user defined most browsers have the accessibility feature a user defined css 5 selector specificity a specific contextual selector heading p overwrites generic definition 6 rule order last rule declaration has a higher priority 7 parent inheritance if a property is not specified it is inherited from a parent element 8 css property definition in html document css rule or css inline style overwrites a default browser value 9 browser default the lowest priority browser default value is determined by w3c initial value specifications specificity edit specificity refers to the relative weights of various rules 18 it determines which styles apply to an element when more than one rule could apply based on the specification a simple selector e g h1 has a specificity of 1 class selectors have a specificity of 1 0 and id selectors have a specificity of 1 0 0 because the specificity values do not carry over as in the decimal system commas are used to separate the digits 19 a css rule having 11 elements and 11 classes would have a specificity of 11 11 not 121 thus the selectors of the following rule result in the indicated specificity selectors specificity h1 color white 0 0 0 1 p em color green 0 0 0 2 grape color red 0 0 1 0 p bright color blue 0 0 1 1 p bright em dark color yellow 0 0 2 2 id218 color brown 0 1 0 0 style 1 0 0 0 examples edit consider this html fragment doctype html html head meta charset utf 8 style xyz color blue style head body p id xyz style color green to demonstrate specificity p body html in the above example the declaration in the style attribute overrides the one in the style element because it has a higher specificity and thus the paragraph appears green to demonstrate specificity inheritance edit inheritance is a key feature in css it relies on the ancestor descendant relationship to operate inheritance is the mechanism by which properties are applied not only to a specified element but also to its descendants 18 inheritance relies on the document tree which is the hierarchy of xhtml elements in a page based on nesting descendant elements may inherit css property values from any ancestor element enclosing them in general descendant elements inherit text related properties but their box related properties are not inherited ...
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