GtkHTML defines a "new" attribute type with:

static const PangoAttrClass html_pango_attr_font_size_klass = {
        PANGO_ATTR_SIZE,
        html_pango_attr_font_size_copy,
        html_pango_attr_font_size_destroy,
        html_pango_attr_font_size_equal
};

The reuse of PANGO_ATTR_SIZE there is *highly* invalid, size attributes
have to be allocated using pango_attr_size_new(). What GtkHTML should
be doing is to add a *separate* custom attribute to the attribute list 
along with the size. And use a custom type registered with
pango_attr_type_register() for the custom attribute.

The problem that came up with Pango-1.8 is that the structure for 
PANGO_ATTR_SIZE was extended from PangoAttrInt to PangoAttrSize which
adds a 'guint absolute : 1' field. That overlaps with the 
GtkHTMLFontStyle style, with unpredictable results. (Some font styles
will show up with smaller pixel-based sizes.)

So, what now:

 - We could declare 1.8.0 "don't use", back out the extension of
   PANGO_ATTR_SIZE and do the Pango API some other way (add a 
   absolute_size attribute type, likely)

   This is definitely the way I would have gone if I had found the
   problem earlier before 1.8.0, but I hate to do it now.

   (We could keep the pango_attr_size_new_absolute() name to maintain
   better, though not absolute, compatibility with 1.8.0)

 - We could just require the GtkHTML bug fix.... the impact on
   people using older GtkHTML isn't huge; just a bit of font
   mis-sizing.

Any other ideas?