This is workaround for Docs improvement/Quesion: How the ‘custom tag’ supposed to work? · Issue #45 · neoclide/coc-html.
In your session *.x.vim file (see Build-in sessions tips) or in command line use:
call coc#source#custom_elements#AddSource(__path__)…now in your html/javascript files the HTML custom elements competitons are available.
There is also function for updating tags coc#source#custom_elements#Update() and function
for opening source file coc#source#custom_elements#OpenSource(__order_of_add__).
vim8:
mkdir -p ~/.vim/pack/coc-custom_elements/start
cd ~/.vim/pack/coc-custom_elements/start
git clone -b main --single-branch https://github.com/jaandrle/coc-custom_elements --depth 1