[dspillett]

> Thus, it falls back to Segoe UI Symbol, which does not have the same fixed width

That seems like a glaring omission to me. If you are rendering fixed-width-per-character text and need to fall back, surely it makes sense to keep to the same character grid even if it does mess up the feel of your negative space somewhat (thin characters having a lot of space around them, wide characters butting into those beside them slightly). You've explicitly asked for text aligned to a grid, either by using a mono-spaced typeface, by using a <pre> tag, or with other relevant CSS choices, the browser should be trying to achieve that.