diff --git a/autoload/emmet/lang/html.vim b/autoload/emmet/lang/html.vim
index 968a55e..8e20af7 100644
--- a/autoload/emmet/lang/html.vim
+++ b/autoload/emmet/lang/html.vim
@@ -216,7 +216,7 @@ function! emmet#lang#html#parseIntoTree(abbr, type) abort
         endif
         if item[0] ==# '['
           let atts = item[1:-2]
-          if matchstr(atts, '^\s*\zs[0-9a-zA-Z-:]\+\(="[^"]*"\|=''[^'']*''\|=[^ ''"]\+\)') ==# ''
+          if matchstr(atts, '^\s*\zs[0-9a-zA-Z_\-:]\+\(="[^"]*"\|=''[^'']*''\|=[^ ''"]\+\)') ==# ''
             let ks = []
 			if has_key(default_attributes, current.name)
               let dfa = default_attributes[current.name]