Three levels introduced by CSS styles

1. Attribute Introduction: Add a style attribute to the label, only applicable to the current label

2.<style> Tag Introduction: Using <style> tag introduction in the current page, only applicable to the current document

3.<link> Tag Introduction: referencing external CSS style sheet files for all documents that introduce this file

There are three different ways to use it, depending on your needs, but use an external style sheet as much as possible unless you have a special situation.

<! DOCTYPE html>