Glyphhanger — a tool to subset and optimize fonts

I've been playing with variable fonts the other day and got excited about transitioning font weights without layout shifts.

But in the end, I had to conclude that a 785kB heavy variable font is too much for my lightweight blog.

Nicolas Friedli pointed out that I can minimize the font's file size down to 322kB by using Glyphhanger — "the web font utility belt".

The Roboto font I've used includes 826 characters and 948 glyphs. My blog is in English, so I'd use less than 10% of these provided characters.

Glyphhanger lets you remove all these unused characters and glyphs.

# subset Roboto to Latin character set
glyphhanger --subset=roboto.woff2 --format=woff2 --LATIN

By subsetting roboto.woff2 to the Latin character set, I cut the number of included font characters and the resulting file size into more than half!

But after inspecting the subsetted font file with Wakamai Fondue, I discovered that I could probably squeeze out some more characters because I doubt I'll use any of these special characters either ÁÂÃÄÅÆÇÈÉ....

# subset Roboto to ASCII character set
glyphhanger --subset=roboto.woff2 --format=woff2 --US_ASCII

By choosing ASCII, I reduced the font to 95 characters and 121 glyphs, resulting in roughly 200kB. Success!

But now I was entering a dangerous territory because glyphs such as “ or … had been excluded, too. I didn't want to add up with mixed or broken characters.

Luckily, Glyphhanger has a solution for this case, too. Use the --whitelist argument to use the ASCII character set and additional glyphs you care about.

# include custom characters
glyphhanger --subset=roboto.woff2 --format=woff2 --US_ASCII --whitelist=…“”

But then, I didn't want to manage this character set manually. And guess what? Glyphhanger also lets you specify a page, evaluates all the used characters, and then subsets a perfect font that doesn't waste any bytes on the wire.

# print used codepoints
glyphhanger https://stefanjudis.com --string
# !"#%'(),-./0123456789:;?@ABCEFGHIJLMNOPQRSTUVWXYabcdefghijklmnoprstuvwxyz{}©—

# evaluate and subset font depending on used characters on a page
glyphhanger https://www.stefanjudis.com/.../ --subset=roboto.woff2 --family="Roboto"
# Subsetting roboto.woff2 to roboto-subset.woff2 (was 765.65 KB, now 56.54 KB)

That's pretty wild!

My little article component that used Roboto to explain a relative font weight axis (GRAD) just got 700kB smaller!

In conclusion: I highly underestimated how many bytes one can save by inspecting and subsetting fonts. The optimized Roboto variable font still includes all the custom axes and fancy variable font features, but especially for this single component, the page has become substantially lighter.

I'm still on the fence if I'll adopt custom fonts on this site, but this tool and trick are nothing but amazing!