How add Font Lock keywords & not clash with other modes?

[address@hidden]

I wrote a very simple mode that simply adds one keyword of 2 regexs to
the Font Lock mode keywords list...

(("\t+"                    0 'highlight)      ; Highlight
TABs.
 ("^.\\{79\\}\\(.*\\)" 1 'highlight))    ; Highlight chars beyond
column 79

I noticed it doesn't work right in HTML mode.

Is there a way to add custom keywords so that they don't clash with
language modes?

(I saw some code where they added their keywords to the *end* of the
keywords list.  Is that the answer?)

Chris