This is a terrible idea, and I wish sites/foundries would abandon it. For fonts with features like ligatures or kerning, it'll break these when the adjacent gl

This is a terrible idea, and I wish sites/foundries would abandon it. For fonts with features like ligatures or kerning, it'll break these when the adjacent glyphs that should interact happen to come from separate sub-fonts. But even for simple fonts that don't have any such features, it can result in terrible rendering because it's possible the font rasteriser (e.g. freetype), which sees the two parts of the font as completely independent entities, will end up applying hinting in quite different ways based on separate analysis of the disjoint glyph subsets. I've seen real-world examples where the end result is that half the glyphs "snap" to one x-height value, and the other half (from the other sub-font) snap to a value that's a pixel taller or shorter. The font rasteriser has no way of knowing that it should have auto-hinted "globally" across the two separate resources.