This is the kind of thing that works really well when written as a filter (i.e. a program that reads data from STDIN and writes it to STDOUT).

#!/usr/bin/perl

use strict;
use warnings;

while (<STDIN>) {
  s/font-size:\s*14pt/font-size: 18pt/ig;
  print;
}
[download]

But you can also write it on the command line:

$ perl -i.bak -pe 's/font-size:\s*14pt/font-size: 18pt/ig;' yourfile.h
+tml
[download]

--
<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg