You could convert your HTML to postscript via HTMLDOC (GPL) and then use Ghostscript.pm (Perl API for Ghostscript) to convert to a ppm. Then convert your ppm to a GIF, which can then be loaded into Image::Magick.
Here is a shell script showing how ghostscript converts a postscript file to a ppm on the command line, you could probably simulate these actions using Ghostscript.pm:
# Call it by putting the .ps file name as first argument
# but without the ".ps" extension.
# Ex: for "Intro_Tbl.ps" use "pstogif Intro_Tbl"
gs -r72x72 -sDEVICE=ppmraw -sOutputFile=$1.ppm << endinput
pnmcrop < $1.ppm | ppmtogif > $1.gif
...This requires both GhostScript and pbmplus
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