Depends on how Mason (which I have no idea about) handles embedding its own stuff in the markup. If it uses processing instructions (<?foo · · · ?>) then HTMLTidy may reindent the opening part of the tag (<?foo) at will and may reindent any following lines as a block, but it won't touch the content. For the majority of languages that means there should be no change, though it might not be what you want.
Unfortunately - and that is the one thing that annoys me about HTMLTidy -, there's no way to specify a list of candidate tags for reindenting or a list of tags not to reindent.
Makeshifts last the longest.