The tag is `<!DOCTYPE html>` and it should be the first line in the file/stream. Doing anything else will enable "quirks mode"[1]. Beyond that, the best thing y

The tag is `<!DOCTYPE html>` and it should be the first line in the file/stream. Doing anything else will enable "quirks mode"[1]. Beyond that, the best thing you can do is avoid features that haven't been standardised yet or that were only standardised recently. [1]: https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode#how_do_browsers_determine_which_mode_to_use https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mod...