/usr/share/vim/vim80/syntax/xhtml.vim is in vim-runtime 2:8.0.1453-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 | " Vim syntax file
" Language: XHTML
" Maintainer: noone
" Last Change: 2003 Feb 04
" Load the HTML syntax for now.
runtime! syntax/html.vim
let b:current_syntax = "xhtml"
" vim: ts=8
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