Strawman: MIME type for fonts

For a little while now, it’s been possible for websites to embed fonts that all major browsers will pick up on. This of course implies fonts being served as HTTP resources. But it turns out that nobody has bothered to assign any of the common font formats a MIME type.1 Fonts being embedded on the web nowadays come in two flavors and three kinds of container: you either have TrueType or PostScript CFF-style outline glyphs, and they are in a bare OpenType (really sfnt) container, or else compressed with either WOFF or EOT. (I am ignoring SVG fonts, which are spottily supported and open several cans of worms that I don’t want to get into right now.) In the future, people might also want to embed TTC font collections, which are also in a sfnt container and could thus also be compressed with WOFF—not sure about EOT there—and bare PostScript Type 1 fonts, but neither of these is supported in any browser at present, as far as I know. There is no official MIME type for any of these combinations; therefore, people deploying fonts over HTTP have been making them up. Without trying very hard, I found real sites using all of: application/ttf, application/otf, application/truetype, application/opentype, application/woff, application/eot, any of the above with an x-prefix, or any of the above in font/ instead of application/ (with or without the x-). There is no top-level font MIME category, making this last particularly egregious.

All of these made-up types work because browsers don’t pay any attention to the content type of a web-embedded font; they look at the data stream, and if it’s recognizably a font, they use it. Such sniffing has historically caused serious problems—recall my old post regarding CSS data theft—so you might expect me to be waving red flags and arguing for the entire feature to be pulled until we can get a standard MIME category for fonts, standard subtypes for the common ones, and browsers to start ignoring fonts served with the wrong type. But I’m not. I have serious misgivings about the whole the server-supplied Content-Type header is gospel truth, content sniffing is evil thing, and I think the font situation makes a nice test case for moving away from that model a bit.

Content types are a security issue because many of the file formats used on the web are ambiguous. You can make a well-formed HTML document that is simultaneously a well-formed CSS style sheet or JavaScript program, and attackers can and have taken advantage of this. But this isn’t necessarily the case for fonts. The sfnt container and its compressed variants are self-describing, unambiguously identifiable binary formats. Browsers thoroughly validate fonts before using them (because an accidentally malformed font can break the OS’s text drawing code), and don’t allow them to do anything but provide glyphs for text. A good analogy is to images: browsers also completely ignore the server’s content-type header for anything sent down for an <img>, and that doesn’t cause security holes—because images are also in self-describing binary formats, are thoroughly validated before use, and can’t do anything but define the appearance of a rectangle on the screen. We do not need filtering on the metadata, because we have filtering on the data itself.

Nonetheless, there may be value in having a MIME label for fonts as opposed to other kinds of binary blobs. For instance, if the server doesn’t think the file it has is a font, shouldn’t it be able to convince the browser of that, regardless of whether the contents of the file are indistinguishable from a font? (Old hands may recognize this as one of the usual rationales for not promoting text/plain to text/html just because the HTTP response body happens to begin with <!DOCTYPE.) The current draft standard algorithm for content sniffing takes this attitude with images, recommending that browsers only treat HTTP responses as images if their declared content type is in the image/ category, but ignore the subtype and sniff for the actual image format. With that in mind, here’s my proposal: let’s standardize application/font as the MIME type for all fonts delivered over the Internet, regardless of their format. Browsers should use only fonts delivered with that MIME type, but should detect the actual format based on the contents of the response body.

I can think of two potential problems with this scheme. First, it would be good if browsers could tell servers (using the normal Accept: mechanism) which specific font formats they understand. Right now, it’s reasonable to insist that browsers should be able to handle either TrueType or PostScript glyph definitions, in either bare sfnt or compressed WOFF containers, and ignore the other possibilities, but that state won’t endure forever. SVG fonts might become useful someday (if those cans of worms can be resolved to everyone’s satisfaction), or someone might come up with a new binary font format that has genuine advantages over OpenType. I think this should probably be handled with accept parameters, for instance Accept: application/font;container=sfnt could mean I understand all OpenType fonts but no others. The other problem is, what if someone comes up with a font format that can’t reliably be distinguished from an OpenType font based on the file contents? Well, this is pretty darn unlikely, and we can put it into the RFC defining application/font that future font formats need to be distinguishable or else get their own MIME type. The sfnt container keeps its magic number (and several other things that ought to be in the file header) in the wrong place, but as long as all the other font formats that we care about put their magic number at the beginning of the file where it belongs, that’s not a problem.

1 To be precise, there is a standard MIME type for a font format: RFC 3073 defines application/font-tdpfr for the Bitstream PFR font format, which nobody uses anymore, except possibly some proprietary television-related products. Bitstream appear to have been trying to get it used for web fonts back in the days of Netscape 4, and then to have given up on it, probably because the font foundries’ attitude was NO YOU CAN’T HAS LICENSE FOR WEBS until just last year.