Many fonts render \u200b <zero width space> and \ufeff <zero width no break space> as the missing glyph. This causes problems for text layout which requires these characters to simply not be rendered. There is no good reason for any TrueType font to not map these Unicodes to glyphs because all TrueType fonts contain a null glyph in glyph code 1.

The font rendering code should make sure that zero width spaces are never rendered even if the font hasn't explicitly mapped them to the null glyph.