General CSS usage skills in advanced tutorials

This article summarizes some common CSS skills to lay the foundation for website reconstruction. I hope you can learn something useful.
I. Use css abbreviations
Using abbreviations can help reduce the size of your CSS file and make it easier to read. For the main rules for css abbreviations, see css basic syntax. II. Define the unit unless the value is 0
Unit that forgets to define the size is a common mistake for beginners of CSS. In HTML, you can write only width = 100, but in CSS, you must give an accurate unit, for example, width: 100px width: 100em. There are only two exceptions. The unit is not defined: the row height and the 0 value. Other values must follow the unit. Note that do not add spaces between values and units. III. Case Sensitive
When CSS is used in XHTML, the element names defined in CSS are case sensitive. To avoid this error, we recommend that all definition names be in lower case.
The values of class and id are also case-sensitive in HTML and XHTML. If you must write them in combination with uppercase and lowercase, make sure that your CSS definition is consistent with the tags in XHTML. 4. Cancel the element limitation before class and id
When you write an element to define a class or id, you can omit the previous element limitation, because ID is unique in a page,