It might be a good idea to go ahead and put all those changes in a single external style sheet and then rely on CSS inheritance to have the correct font name and size cascade down. It's also a good idea to have multiple fonts named, in case there is no equivalent on the displaying system.

# font-family: arial, helvetica, sans-serif;

#!/usr/bin/perl
use strict;
use warnings;

while (<>) {
   s/font-family:\s+'*Arial'*/font-family: Helvetica, sans-serif;/g;
   s/font-size:\s+'*10pt'*/font-size: 18pt;/g;
   print;
}
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