Is there a standard descent that fonts use as a proportion of their font height? It seems like their is as if you use multiple common fonts in the same sentence, all of their baselines appear to align.
Is there a standard descent that fonts use as a proportion of their font height?
There is no standard. The vertical metrics will vary from typeface to typeface, and although most will fall within a vaguely similar range, there is no absolute standard.
It seems like their is as if you use multiple common fonts in the same sentence, all of their baselines appear to align.
The reason why is right there in your question. Fonts are drawn from their baseline. So none of the other vertical metrics matter; the descenders, ascenders, cap-height, x-height (and even font size) can all be vastly different and the fonts will still be drawn on the same baseline.
Take a few random fonts as an example...
They all sit on the same baseline (the orange line), but none of the descenders align (the magenta lines). A lot are similar, but none are exactly the same.
As for actual values (although that's pretty much besides the point now)...
Assuming a UPM (units-per-em; the units by which digital fonts are designed and their metrics defined) of 1000 and a baseline sitting at 0, most non-cursive/script fonts have descenders in the range between −200 and −300. That should work out at roughly 20–30% of the typeface's height (although with digital type, the overall "height" of a font isn't well defined; glyphs can easily extend far outside any defined metrics).
That is a very rough estimate (based on experience and a handful of fonts I just inspected) and by no means a standard or guaranteed range, but it's a good starting estimate.
Most any given built font (as opposed to drawn, and novelty styles) has common points of reference as part of its design. These metrics are more or less standard and have names that correspond to their function.
Some font metrics refer to every font; but, these are not the same for ALL fonts of a given size.
Only font size metric has been standardized by international agreement for convenience.
- font size: The height of the imaginary box formed by averaging all the font glyphs measured in points.
- baseline: The line upon which all font glyphs appear to sit.
- x-height: The line for the average body height of lower-case font glyphs
- descender line: The limit below the base line for bottom parts of lower-case font glyphs
- ascender line: The upper limit for lower-case font glyphs
- cap height: The upper limit for the upper-case font glyphs
Depending on the mood of the designer, the descender can be most any proportion of the font. It might not vary among common fonts.
For mixed fonts, there's a good chance that the x-height would be different. Different x-heights at any given size would affect the proportion of ascenders and/or descenders. The effect is more apparent in larger sizes.
There is no standard proportion for these. Software for designing typefaces has the descender line variable. The baseline is not variable while all other metrics are variable according to the whim of the designer.