disclaimer: the completion setup is for vsnip but I think you can easily adapt it to other completion engines.
so the nice thing about my setup is that it only sets up html suggestion for specific projects using the power of excr files (.nvim.lua in this case). (it also works for CSS suggestions if you are into css-in-rust stuff)
First you want to have this in your lua config to enable projet-specific config files.
vim.o.exrc = true