Oh, okay. I decided to take a whack at it. Normal HTML caveats apply, edge cases and all that, but this should work pretty well. It even tries to determine if the font tag is alone in a parent so that the style can be bumped to the parent. In cases like-

<td><font size="6">Pretty BIG</font></td>
# we'll get (updated typo in "larg")
<td style="font-size:x-large">Pretty BIG</td>
[download]

Barely tested though I think it's pretty solid. Improvements welcome. (Update: tested a bit and found 2 bugs, fixed now with inline test case.)

use strict;
use warnings;
no warnings 'uninitialized';

use HTML::TreeBuilder;
use HTML::Element;

my %size_convert = qw(
                      1  xx-small
                      2  x-small
                      3  small
                      4  medium
                      5  large
                      6  x-large
                      7  xx-large
                     -6  40%
                     -5  50%
                     -4  60%
                     -3  70%
                     -2  80%
                     -1  90%
                     -0  100%
                     +0  100%
                     +1  120%
                     +2  140%
                     +3  160%
                     +4  180%
                     +5  200%
                     +6  250%
                     );

#  Program proper
#=====================================================================

my $file = shift;
$file or warn "No file given, using test HTML!\n";
$file = \*DATA;

my $tree = HTML::TreeBuilder->new(); # empty tree

$tree->parse_file($file);

for my $node ( $tree->descendants() ) {
    next unless $node->tag() eq 'font';

    my $font_style = extract_style( $node );

    $font_style->{"font-size"} = $size_convert{ $node->attr('size') }
        if $size_convert{ $node->attr('size') };

    $font_style->{"font-family"} = $node->attr('face')
        if $node->attr('face');

    $font_style->{color} = $node->attr('color')
        if $node->attr('color');

    if ( $node->parent() and
         $node->parent->content_list() == 1 ) {

        my $parent = $node->parent;
        my $parent_style = extract_style( $parent );

        $font_style = {
                       %{$parent_style},
                       %{$font_style},
                      };

        my $style = generate_style_string($font_style);

        $parent->attr("style", $style);
        $parent->push_content( $node->as_text() );
        $node->delete();
    }
    else # replace font with span
    {
        my $style = generate_style_string($font_style);
        my $span = HTML::Element->new('span', style => $style );
        $node->replace_with($span);
    }
}

print $tree->as_HTML();

exit 0;

#  Subroutines
#=====================================================================
sub generate_style_string {
    my $style_hash = shift;
    my @style;
    while ( my ( $k, $v ) = each %{$style_hash} ) {
        push @style, join(":", $k, $v);
    }
    return join "; ", @style;
}

sub extract_style {
    my $node = shift;
    my %style;
    %style = map { split /:/ } split /;/, $node->attr('style')
        if $node->attr('style');
    return \%style
}

__DATA__
<table><tr>
<td style="border:solid 1px black;"><font size="+2">Some text</font></
+td>
</tr>
</table>

<p>Something with <font color="#ffee98" size="7" face="Times">3 style
points</font></p>

<p style="color:#aaa"><font color="red" face="Times">3 style points</f
+ont></p>
[download]