I'm trying to have the command
let b:match_words='<:>,<\@<=\([^/][^ \t>]*\)[^>]*\%(>\|$\):<\@<=/\1>'
run every time I open an html file. I tried putting the line
autocmd FileType html let b:match_words='<:>,<\@<=\([^/][^ \t>]*\)[^>]*\%(>\|$\):<\@<=/\1>'
in a file named html.vim in both my ftdetect and ftplugin folders and nothing happened. How do I have the command run everytime I'mm in an html file?
The command is to change the matching behavior of matchit btw.
In general, your autocmd is alright; the problem is that you're trying to redefine the b:match_words definition done in $VIMRUNTIME/ftplugin/html.vim, so the execution order becomes important.
The place for these customizations is in the after directory, i.e. ~/.vim/after/ftplugin/html.vim; just create a new file and put the :let command in there.
You can observe the sequence of sourced scripts via :scriptnames. In other cases, when you're not overriding default behavior, the :autocmd FileType is alright, but I prefer putting these (e.g. custom mappings) into ~/.vim/ftplugin/html_mymappings.vim, as it provides better separation and helps keeping your .vimrc short and understandable.
The ftdetect subdirectory is for filetype detection, i.e. inspecting file path / name / contents to determine the correct filetype. It doesn't apply here, as the filetype is html.