What is the purpose of Docntye in HTML what is the pattern and what business hooks are there

1 in-progress validation of the document 2 determines the rendering mode of the browser

Standard mode is strict,

Weird patterns and promiscuous patterns are not allowed, old, wrong

Business hooks with progressive enhancements elegant downgrade

Incremental increase is mainly for the bottom version of the browser for page construction, to ensure basic execution. Interactive and improved for high-version browsers, additional features

Gracefully downgrade, build features from the start, and then compatible with the bottom version of the browser

Triangle

width:50px

height:50px

Backround:red

Backround-color:red

Border-top;2 Solid Red

Border-right

border-bottom:20px Solid Transparent

border-left:20px

}

What is the purpose of Docntye in HTML what is the pattern and what business hooks are there