1. Tag selector
Directly use html tags as selectors, which can be html tags such as p div strong
2. id selector
Set the id attribute for a tag, you can use #id{} to set the attribute in css
Third, the class selector
Set the class attribute class="cName" to a tag. You can use .cName{} in css to set the style of this tag
The group selector
Multiple tags can use the same style sheet, which can be combined by the above three
5. Descendant selector
p strong{} indicates that this style will be applied to the strong under the p tag