Note that the font renderer is NOT responsible for actually understanding the dom tree and converting that to glyph compositions. It is just there to make sure that the metrics and other size info stored in the font gets used to correctly draw glyphs and compose them with existing gtklayout boxes. This is the angle at which the symbols in a font are slanted and allows the program to move superscript indices to the right and subscript indices to the left. Such information is available in the TFM metric format as described in appendix G of the TeXbook. For roots, this is more complicated.
Symbol fonts contain so many parameters because they contain operator glyphs.