Lesson03_02 Style Rule Selector_Basic Tutorial

样式规则选择器

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• HTML selector

  就是HTML标签做为selector,这样在所有CSS应用的网页中,所有HTML标签都会按照所定义的语句来进行显示.

• Class selector
  要将一种HTML标签所创建的各个网页元素分成几类,则要将这个HTML标签的class设置成为不同的值:
  例如:
  

  ---

  p1

  
  

  p2

  
  

  p3

  样式定义如下:
  

  注:如果在样式表定义的时候,selector直接以(.)后接类名,这条样式规则就适用于所有class=该类名,的所有HTML标签. 即把class selector分为了两类:
  
  
  1. 关联class selector: HTML标签名.类名(可以为同一元素定义不同的规则).
  2. 独立class selector: .类名(可以为不同的元素定义相同的样式规则).
• ID selector ID属性用来定义某一特定的HTML元素,一个网页文件中只能有一个元素使用某一ID值.

  网页中:
  text here

  样式定义如下:
  

• Associative selectors refer to a string consisting of two or more single selectors separated by a space
  For example:
  p em {background: yellow}
  The "p em" It is the association selector, which means that the background of the content between the

  tag pair in the

  tag pair is yellow, and the content in the tag pair from other places is not affected.
  Note: Association selection is defined Rules take precedence over rules defined by a single selector.

• Combined selectors In order to reduce repeated declarations in style sheets, several selectors can be combined in one style rule definition.
  Example: H1, H2, H3, H4, H5, H6, TD{color:red}
• Pseudo-element selector refers to a way of defining various states of the same HTML element and part of the content it contains.
  The format is as follows: HTML element: pseudo-element periodic {attribute: value}

  Commonly used pseudo-elements are as follows

  Element name	Function
  A:active	The state when a hyperlink is selected
  A:hover	The state when the cursor moves to the hyperlink
  A:link	The state without any action
  A:visited	Visited hyperlink status
  P:first-line	The first line of text in a paragraph
  P:first-letter	The first letter in the paragraph

  
  The class selector can be used with pseudo-elements:
  HTML element.class name :pseudo-element {attribute:value}