CSS-Pseudo-elements (偽元素)與 Pseudo-classes (偽類)
兩個冒號(::)是偽元素Pseudo-element，一個冒號是(:)偽類Pseudo-class
Pseudo-class顧名思義其特性就像是selector在操作class或是element一般的使用，
也可跟jQuery的filter配合，
例如：$( "li" ).filter( ":even" ).css( "background-color", "red" );
一、Pseudo-class
1、:link、:visited、:hover、:active
此範例為針對超連結被點選前(紅)、訪問過(綠)、滑鼠移到超連結上(紫)、滑鼠點選時(藍)
:link、:visited、:hover、:active須依先後順序放才能有正確效果
2、:first-child的應用
2-1、:first-child基本型
選取該元素在父節點底下集合中排列第一位的指定元素，
注意如果有非指定元素排列在第一位，則指定元素將不會被選到。
2-2、p i:first-child將每個<p>tag底下的第一個<i>tag的字體變成藍色
2-3、p:first-child i將第一個<p>tag的所有的<i>tag字體變成藍色
3、更多其他用法的有
| Selector | Example | Example description |
|---|---|---|
| :active | a:active | Selects the active link |
| :checked | input:checked | Selects every checked <input> element |
| :disabled | input:disabled | Selects every disabled <input> element |
| :empty | p:empty | Selects every <p> element that has no children |
| :enabled | input:enabled | Selects every enabled <input> element |
| :first-child | p:first-child | Selects every <p> elements that is the first child of its parent |
| :first-of-type | p:first-of-type | Selects every <p> element that is the first <p> element of its parent |
| :focus | input:focus | Selects the <input> element that has focus |
| :hover | a:hover | Selects links on mouse over |
| :in-range | input:in-range | Selects <input> elements with a value within a specified range |
| :invalid | input:invalid | Selects all <input> elements with an invalid value |
| :lang(language) | p:lang(it) | Selects every <p> element with a lang attribute value starting with "it" |
| :last-child | p:last-child | Selects every <p> elements that is the last child of its parent |
| :last-of-type | p:last-of-type | Selects every <p> element that is the last <p> element of its parent |
| :link | a:link | Selects all unvisited links |
| :not(selector) | :not(p) | Selects every element that is not a <p> element |
| :nth-child(n) | p:nth-child(2) | Selects every <p> element that is the second child of its parent |
| :nth-last-child(n) | p:nth-last-child(2) | Selects every <p> element that is the second child of its parent, counting from the last child |
| :nth-last-of-type(n) | p:nth-last-of-type(2) | Selects every <p> element that is the second <p> element of its parent, counting from the last child |
| :nth-of-type(n) | p:nth-of-type(2) | Selects every <p> element that is the second <p> element of its parent |
| :only-of-type | p:only-of-type | Selects every <p> element that is the only <p> element of its parent |
| :only-child | p:only-child | Selects every <p> element that is the only child of its parent |
| :optional | input:optional | Selects <input> elements with no "required" attribute |
| :out-of-range | input:out-of-range | Selects <input> elements with a value outside a specified range |
| :read-only | input:read-only | Selects <input> elements with a "readonly" attribute specified |
| :read-write | input:read-write | Selects <input> elements with no "readonly" attribute |
| :required | input:required | Selects <input> elements with a "required" attribute specified |
| :root | root | Selects the document's root element |
| :target | #news:target | Selects the current active #news element (clicked on a URL containing that anchor name) |
| :valid | input:valid | Selects all <input> elements with a valid value |
| :visited | a:visited | Selects all visited links |
Tip1：
另外一點須注意的是CSS元素選取的方法雖然跟jQuery差不多，
但還是有不一樣的地方，例如jQuery有p:first的語法，
但pseudo-class卻是沒有這種語法喔。
Tip2：
在撰寫CSS樣式時須注意Pseudo-class跟Selector中間要留空白，在某瀏覽器可能無作用。
<!DOCTYPE html> <html> <head> <title>hover</title> <meta charset="utf-8" /> <style> body { background: #000000; margin: 0; } ul { list-style-type: none; margin: 0; padding: 0; } li { display: inline-block; } .cover { position: absolute; top: 0; z-index:-1; } .color { opacity: 0.1; z-index:1; } .color :hover { opacity: 1; } .menu3 :hover { opacity: 1; } </style> </head> <body> <ul> <li class="menu3"> <div class="color"> <img src="http://i.imgur.com/LBJxiKM.png" /> </div> <div class="cover"> <img src="http://i.imgur.com/vyJOnbJ.png" /> </div> </li> </ul> </body> </html>
二、Pseudo-element
1、p::first-line表示對p tag 的第一行do something
2、::first-letter表示對某tag的第一的字母do something
3、::before表示對某tag「裡面的前面」do something
<!DOCTYPE html> <html> <head> <style> h1 { border: 1px solid red; } h1::before { content: url(http://www.w3schools.com/css/smiley.gif); } </style> </head> <body> <h1>This is a heading</h1> <p>The ::before pseudo-element inserts content before an element.</p> <h1>This is a heading</h1> <p><b>Note:</b> IE8 supports the content property only if a !DOCTYPE is specified.</p></body>
</html>
雖然現在的瀏覽器對偽元素就算寫一個冒號也可以正常運作，不過為了方便區分，用兩個冒號還是比較好的。
4、更多其他用法的有
| Selector | Example | Example description |
|---|---|---|
| ::after | p::after | Insert content after every <p> element |
| ::before | p::before | Insert content before every <p> element |
| ::first-letter | p::first-letter | Selects the first letter of every <p> element |
| ::first-line | p::first-line | Selects the first line of every <p> element |
| ::selection | p::selection | Selects the portion of an element that is selected by a user |
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