HTML tutorial: DOCTYPE abbreviation_HTML/Xhtml_Web page production

When writing HTML code, you must write DOCTYPE in the first line. DOCTYPE is usually very long, and people are too lazy to remember it, so they basically just copy what they have written before. But today I learned about the abbreviation of DOCTYPE. If your DOCTYPE is as follows.
When writing HTML code, DOCTYPE must be written in the first line. DOCTYPE is usually very long, and people are too lazy to remember it, so they basically just copy what they have written before. But today I learned about the abbreviation of DOCTYPE. If your DOCTYPE is as follows:
<br> can be replaced by the following abbreviations: <br><code>
Simple and easy to remember, so you don’t need to copy it in the future. BTW, I wanted to try to draw inferences , but it failed and the page became "mixed mode". If you have any other tips, please provide them.