There’s several things that all need to be in place. Just to summarize them all in one location:
Set the following option:
:filetype indent on
:set filetype=html # abbrev - :set ft=html
:set smartindent # abbrev - :set si
Then either move the cursor to the top of the file and indent to the end: gg
=G
Or select the desired text to indent and hit = to indent it.