Searching with one of those terms works just fine:
- [html] [css] 
But in some other cases, when I remove a tag (like double click the tag, hit delete, hit enter) the empty braces "" are replaced by one or more plus signs like this:
- [html]  becomes [html+]
-  [css] becomes [+css]
Is this a bug that in some cases the empty braces "" don't get filtered?