The CSS rule set consists of selectors and declaration blocks to define the style of HTML elements. 1. The selector specifies the target element, such as a tag, class, or ID; 2. Declare the block to contain attributes and values to control the appearance of the element. For example: p { color: blue; font-size: 16px; } means to select the paragraph and set the text color and font size. Master these two parts to write effective CSS styles.
The CSS rule set is a core component of a style sheet, which consists of selectors and declaration blocks to define the appearance and layout of HTML elements. To understand how CSS works, you must first understand its basic structure.
Selector
The function of the selector is to indicate which HTML elements you want to apply styles to. It can be a tag name, class, ID, or it can be a more complex combination.
for example:
-
p: Select all paragraph elements -
.highlight: Select all elements withhighlightclass -
#header: Select the element with IDheader
You can also use a combination selector, such as div p , which means selecting all paragraphs inside div .
Some common tips:
- Class selectors can be reused and are suitable for sharing styles for multiple elements.
- The ID selector is highly unique and suitable for elements that only appear once on the page.
- Use commas to separate multiple selectors, such as
h1, h2, h3you can set the title style uniformly
Declaration Block
A declaration block is a set of attributes and values wrapped in braces {} to define the style of the selected element.
For example:
p {
color: blue;
font-size: 16px;
} Here color and font-size are attributes, followed by the colon, and each declaration ends with a semicolon.
Some points to note:
- The attribute must be a legal CSS attribute name and cannot be written casually.
- The value must comply with the specifications of this attribute, such as
coloraccepts color values,font-sizeaccepts length units - If a declaration is invalid, the browser will usually ignore it, but it will not affect the execution of other declarations
Complete rule set structure
A complete CSS rule set looks like this:
selector {
property1: value1;
property2: value2;
}Each line is a declaration that controls the visual representation of the selected element.
For example:
.button {
background-color: #007BFF;
border: none;
padding: 10px 20px;
} This code applies background color to all elements with .button class, removes borders, and adds inner margins.
Small details in actual use
Sometimes you may see some less common writing styles, such as omitting the semicolon of the last declaration, or using comments to organize your code. These are all permitted, but for maintainability it is recommended to maintain a consistent format.
Also, although the browser is not strict with the format, good indentation and line breaks make it easier for you to read and debug the code.
Basically that's it. As long as you master the two core parts of the selector and the declaration block, you can start writing clear and effective CSS rules.
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