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Merge pull request #405 from mvalipour/master
[css/en] CSS article in english
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---
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language: css
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contributors:
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- ["Mohammad Valipour", "https://github.com/mvalipour"]
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---
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In early days of web there was no visual elements, just pure text. But with the
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further development of browser fully visual web pages also became common.
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CSS is the standard language that exists to keep the separation between
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the content (HTML) and the look-and-feel of web pages.
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In short, what CSS does is to provide a syntax that enables you to target
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different elements on an HTML page and assign different visual properties to them.
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Like any other language, CSS has many versions. Here we focus on CSS2.0
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which is not the most recent but the most widely supported and compatible version.
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**NOTE:** Because the outcome of CSS is some visual effects, in order to
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learn it, you need try all different things in a
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CSS playground like [dabblet](http://dabblet.com/).
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The main focus of this article is on the syntax and some general tips.
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```css
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/* comments appear inside slash-asterisk, just like this line! */
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/* ####################
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## SELECTORS
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####################*/
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/* Generally, the primary statement in CSS is very simple */
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selector { property: value; /* more properties...*/ }
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/* the selector is used to target an element on page.
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You can target all elments on the page! */
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* { color:red; }
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/*
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Given an element like this on the page:
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<div class='some-class class2' id='someId' attr='value' />
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*/
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/* you can target it by it's class name */
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.some-class { }
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/*or by both classes! */
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.some-class.class2 { }
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/* or by it's tag name */
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div { }
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/* or it's id */
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#someId { }
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/* or by the fact that it has an attribute! */
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[attr] { font-size:smaller; }
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/* or that the attribute has a specific value */
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[attr='value'] { font-size:smaller; }
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/* start with a value*/
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[attr^='val'] { font-size:smaller; }
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/* or ends with */
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[attr$='ue'] { font-size:smaller; }
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/* or even contains a value */
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[attr~='lu'] { font-size:smaller; }
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/* and more importantly you can combine these together -- there shouldn't be
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any space between different parts because that makes it to have another
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meaning.*/
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div.some-class[attr$='ue'] { }
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/* you can also select an element based on how it's parent is.*/
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/*an element which is direct child of an element (selected the same way) */
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div.some-parent > .class-name {}
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/* or any of it's parents in the tree */
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/* the following basically means any element that has class "class-name"
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and is child of a div with class name "some-parent" IN ANY DEPTH */
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div.some-parent .class-name {}
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/* warning: the same selector wihout space has another meaning.
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can you say what? */
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div.some-parent.class-name {}
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/* you also might choose to select an element based on it's direct
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previous sibling */
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.i-am-before + .this-element { }
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/*or any sibling before this */
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.i-am-any-before ~ .this-element {}
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/* There are some pseudo classes that allows you to select an element
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based on it's page behaviour (rather than page structure) */
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/* for example for when an element is hovered */
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:hover {}
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/* or a visited link*/
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:visited {}
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/* or not visited link*/
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:link {}
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/* or an input element which is focused */
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:focus {}
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/* ####################
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## PROPERTIES
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####################*/
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selector {
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/* Units */
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width: 50%; /* in percent */
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font-size: 2em; /* times current font-size */
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width: 200px; /* in pixels */
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font-size: 20pt; /* in points */
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width: 5cm; /* in centimeters */
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width: 50mm; /* in millimeters */
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width: 5in; /* in inches */
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/* Colors */
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background-color: #F6E /* in short hex */
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background-color: #F262E2 /* in long hex format */
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background-color: tomato /* can be a named color */
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background-color: rgb(255, 255, 255) /* in rgb */
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background-color: rgb(10%, 20%, 50%) /* in rgb percent */
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background-color: rgba(255, 0, 0, 0.3); /* in semi-transparent rgb */
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/* Images */
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background-image: url(/path-to-image/image.jpg);
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/* Fonts */
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font-family: Arial;
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font-family: "Courier New"; /* if name has space it appears in double-quote */
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font-family: "Courier New", Trebuchet, Arial; /* if first one was not found
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browser uses the second font, and so forth */
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}
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```
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## Usage
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Save any CSS you want in a file with extension `.css`.
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```xml
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<!-- you need to include the css file in your page's <head>: -->
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<link rel='stylesheet' type='text/css' href='filepath/filename.css' />
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<!-- you can also include some CSS inline in your markup. However it is highly
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recommended to avoid this. -->
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<style>
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selector { property:value; }
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</style>
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<!-- or directly set CSS properties on the element.
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This has to be avoided as much as you can. -->
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<div style='property:value;'>
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</div>
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```
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## Precedence
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As you noticed an element may be targetted by more than one selector.
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and may have a property set on it in more than one.
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In these cases, one of the rules takes precedence over others.
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Given the following CSS:
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```css
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/*A*/
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p.class1[attr='value']
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/*B*/
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p.class1 {}
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/*C*/
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p.class2 {}
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/*D*/
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p {}
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/*E*/
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p { property: value !important; }
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```
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and the following markup:
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```xml
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<p style='/*F*/ property:value;' class='class1 class2' attr='value'>
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</p>
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```
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The precedence of style is as followed:
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Remember, the precedence is for each **property**, not for the entire block.
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* `E` has the highest precedence because of the keyword `!important`.
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It is recommended to avoid this unless it is strictly necessary to use.
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* `F` is next, because it is inline style.
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* `A` is next, because it is more "specific" than anything else.
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more specific = more specifiers. here 3 specifiers: 1 tagname `p` +
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class name `class1` + 1 attribute `attr='value'`
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* `C` is next. although it has the same specificness as `B`
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but it appears last.
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* Then is `B`
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* and lastly is `D`.
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## Compatibility
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Most of the features in CSS2 (and gradually in CSS3) are compatible across
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all browsers and devices. But it's always vital to have in mind the compatiblity
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of what you use in CSS with your target browsers.
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[QuirksMode CSS](http://www.quirksmode.org/css/) is one of the best sources for this.
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## Further Reading
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* [Understanding Style Precedence in CSS: Specificity, Inheritance, and the Cascade](http://www.vanseodesign.com/css/css-specificity-inheritance-cascaade/)
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* [QuirksMode CSS](http://www.quirksmode.org/css/)
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* [Z-Index - The stacking context](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context)
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