Add option to prevent fallback fonts to be used for missing characters

[carbon-starlight]

Describe the bug
I was hoping this page would help me with figuring out what characters are supported by certain fonts and how are they displayed, but it seems to have the same issue as my local software, terminal and browser editors: if a glyph is unknown, it is pulled from another font (Unifont, Noto or alike).

I believe it is a bug as it greatly diminishes value of the resource in terms of helping to figure out what characters are supported, (e.g. 𐇺𜱕Ӝ𓉣𔋵 seem to be supported with every font) and skews up box drawing, leaving us wonder if a shifted ANSI art is a problem with a font or font replacement (sometimes spaces may be of another size because they are supported and, say, box drawings or type pieces are pulled from backup font).

If stopping the borrowing is not possible, it would be nice to at least highlight them.

To Reproduce
Paste a string like 𐇺𜱕Ӝ𓉣𔋵 to editor for font that does not support it.

Expected behavior
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Screenshots
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Environment (please complete the following information):

OS release

NAME="Fedora Linux"
VERSION="41 (Workstation Edition)"
RELEASE_TYPE=stable
ID=fedora
VERSION_ID=41
VERSION_CODENAME=""
PLATFORM_ID="platform:f41"
PRETTY_NAME="Fedora Linux 41 (Workstation Edition)"
ANSI_COLOR="0;38;2;60;110;180"
LOGO=fedora-logo-icon
CPE_NAME="cpe:/o:fedoraproject:fedora:41"
DEFAULT_HOSTNAME="fedora"
HOME_URL="https://fedoraproject.org/"
DOCUMENTATION_URL="https://docs.fedoraproject.org/en-US/fedora/f41/"
SUPPORT_URL="https://ask.fedoraproject.org/"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Fedora"
REDHAT_BUGZILLA_PRODUCT_VERSION=41
REDHAT_SUPPORT_PRODUCT="Fedora"
REDHAT_SUPPORT_PRODUCT_VERSION=41
SUPPORT_END=2025-12-15
VARIANT="Workstation Edition"
VARIANT_ID=workstation

Browser: Chrome Version 139.0.7258.138 (Official Build) (64-bit)

[braver]

Oh, interesting. It is simply how the underlying system down to the OS works. It's not a bug in that sense, but I do see how that affects your use case. In some cases I've gone so far as to reduce the supported character sets in some fonts, because they're simply too large to make sense in a web context. Or if there is a reduced set I would take that for the same reason.

The only way I can think of solving the technical issue would be to provide a fake custom font that says it has all possible characters, but all of them are "tofu". I don't know that such a thing exists. I also don't believe there is a way I can say in CSS "don't fall back on other fonts", simply because that's not normally desirable.

And in any case, right now, this project was never set up to do what you want. So if we find a way to provide the functionality, we'd still have to audit all existing font files to ensure they're not subsetted somehow. Not to say it wouldn't be great if it did, it would be nice to be able to see if power line symbols and box drawing characters are supported in the font, or Greek, or Arabic, etc.

[carbon-starlight]

I found a StackOverflow post discussing this issue: https://stackoverflow.com/questions/13844927/prevent-browsers-from-using-default-fallback-fonts

They found two such fallback fonts: one by Adobe (but OFL-licensed) that shows ⌧ in place of every character, one by SIL that is heavier and shows codepoint number, but because number of distinct glyphs in a font file is limited to 65536, it covers only BMP. I could not find a non-BMP version.

IDK whether you can adjust width of these boxes to whatever is width of characters in a given font and if it would be desirable.

In some cases I've gone so far as to reduce the supported character sets in some fonts, because they're simply too large to make sense in a web context.

Would be nice to at least have a warning on this.

[braver]

Would be nice to at least have a warning on this.

Describing coverage of fonts is not super easy anyway, nor do all fonts provide that information, so I've tried to avoid the topic.

one by Adobe (but OFL-licensed) that shows ⌧ in place of every character,

That's promising actually. With that I can at least make it a toggle or something.

Let's see if I can remember, or find in the git logs, which fonts I've actively subsetted 😅