Minimum¹ valid HTML file, did you say? Here you go:
<!doctype html><html lang><title></title>
(Tested with https://validator.w3.org/nu/#textarea)
I mean, needless to say, don’t do this. Just because you can doesn’t mean you should :)
¹ Minimum number of characters (not bytes) without errors or warnings, that is. If you don’t care about warnings, you can remove <html>. If you want the minimum number of bytes, replace the emoji with a dot or something ;)