Trying to create a lightweight fallback font with a substitute all glyphs feature.

For a web app I need a way to prevent that a browser falls back to another font if my web font doesn't include a character. It seems the only way to do this is to add another font to the fontstack which includes "all" possible characters .

There are already existing fallback fonts, but those are more debug helpers as they show the codepoint as number, therefore they are much to heavy (>2MB).

The fallback font for my use case should just show something like a box to signal a missing character.

My idea was to generate a simple font with only one glyph and apply a feature file which will substitute all glyphs with this one.

My script for fontforge:

import fontforge
import fontTools.feaLib.builder as feaLibBuilder
from fontTools.ttLib import TTFont

font_name = 'maeh.ttf'
font = fontforge.font()
glyph = font.createChar(33, "theone")
pen = glyph.glyphPen()
pen.moveTo((100,100))
pen.lineTo((100,500))
pen.lineTo((500,500))
pen.lineTo((500,100))
pen.closePath()

for i in range(34, 99):
    glyph = font.createChar(i)
    glyph.width=10

font.cidConvertTo('Adobe', 'Identity', 0)  # doesn't make a difference

font.generate(font_name)

font = TTFont(font_name)
feaLibBuilder.addOpenTypeFeatures(font, 'fallback.fea')
font.save("fea_"+font_name)

My feature file:

languagesystem DFLT dflt;

@all=[\00035-\00039];
#@all=[A-Z]   this works 

feature liga {
    sub @all by theone;
} liga;

For testing I target not 'all' glyphs, only the range from 33-99.

But the above results in a

KeyError: ('cid00037', 'SingleSubst[0]', 'Lookup[0]', 'LookupList')

with changing numbers for cid00037.

If I use the out commented class definition A-Z from the Feature file it works, so this approach doesn't seem to be completely wrong. But how to create a class this way which includes 'all' possible 'glyphs'.

Why can't fonttools find the glyphs if I specify the range in CID notation? Is there another way to crate a class for the OpenType feature file which includes all glyphs?

I use 'liga' in the feature file as this seems to be the most supported tag. Is there a more appropriated one.