How to edit "mixed" (e.g. HTML/JavaScript) files?

[kj <socyl@987jk.com.invalid>]

It often happens that a single file must contain code corresponding
to two different syntaxes.  For example, an HTML file with embedded
JavaScript or CSS.

Is there a way to *locally* override the mode of a file and replace
it with a different mode?

If not, is there a good way to deal with such situations (other
than avoiding them altogether)?

Thanks!

kj

-- 
NOTE: In my address everything before the first period is backwards;
and the last period, and everything after it, should be discarded.