Some things are always mentioned when it comes to web standards, that is the importance of separation of structure and performance.
Understanding the differences between structure and performance at the beginning can be difficult, especially if you are not used to thinking about the semantic structure of a document.
However, it is important to understand this because when structure and performance are separated, it is easy to use CSS documents to control performance.
A structure is composed of the main part of the document, plus semantic and structured markers.
Expression is a style you give to content. In most cases, the performance is what the document looks like, but it can also affect how a document sounds--after all, not everyone uses an image-based browser.
Separate structure and expression as much as possible.
In theory, you should use an HTML document to save the content and structure, and use a CSS document to control the performance of the entire document.
Unseparated websites of performance and structure are still very common today.
So we just started to learn to develop good habits, put them in separate files, and XHTML files can be linked and shared external CSS and JavaScript documents.