[Help] How do I send Gmail emails in HTML format?

The text editor of Gmail mail can implement rich formatting. With these format setting buttons, I can easily write a seemingly good email. However, not always do I want to write emails in the text editor that comes with Gmail. More often, I want to design emails in other HTML page editors, then send it out through Gmail. I searched everywhere in my Gmail mailbox and didn't find out how to set the function to edit HTML in Gmail. Please help me solve this problem. Thank you!

Update @ 09: 44: Open the edited HTML page in IE (or another browser) and copy all (or select the desired part, paste it in the Gmail editor. OK! No comprehensive test is available. I do not know whether JavaScript scripts and CSS styles can be correctly executed!
Update @ 09: 26: According to the update method on the 24 th, only HTML issues can be solved, but JS and CSS are still a problem. After repeated attempts, we still cannot execute JS and CSS in the email. It seems that we still need to find a stable space to upload JS and CSS to solve this problem. However, I don't know if <link/> in Update @ 19: 08: What if the user sets the email address to [display in plain text format? Thanks to Lao Kang for asking such a question. Write down...