See vuejs/vetur#232 (comment)

In html files, we use the html parser to figure out the cursor is inside the style tag and then use css instead of html abbr

In non html files that need html like abbreviations, the parsing is not available out of the box and so the emmet extension doesn't realize to use css instead html abbr inside the html tag

Update:
See #29114 (comment) for a proposal that could solve this issue by passing the responsibility of figuring out scope to language specific extensions