FF90: Add info about EXIF intrinsic size (basic)

[hamishwillee]

In FF90 Firefox now respects EXIF information about the intrinsic size of an image (see #6240). Essentially this replaces the HTTP Content-DPR header, which allowed a server to specify the intrinsic size of an image. This is useful because (among other reasons) it means a server can send a low quality placeholder image if needed and it will render with exactly the same layout as the original - so nothing gets stuff up when you lazily load the proper one.

This is bare minimal (I hope) docs to capture the fact of this change, without providing much of the detail. I'm hoping perhaps we can get this in for the release and then iterate following discussion. What it does is:

• State the change in the release notes, with a link to the bug and the explainer.
• Provide a glossary doc for EXIF - just so I don't have to explain it again and have a place to link to specs etc.
• Provide a bit of info about what intrinsic size "means" for images. The spec really doesn't cover this at all to my mind https://www.w3.org/TR/css-sizing-3/#intrinsic-sizes

More generally what do we need?

• MDN does not (IMO) properly/well define the meaning of a the various terms used around images and image sizes - specifically there is no one clear place where you can go to understand what the intrinsic size really means and how other properties of an image and device relate (device pixels, CSS pixels, pixel density, etc). I think everyone who has done this before already had a good understanding because I'm finding it very confusing.
• The EXIF changes only become more relevant as the HTTP Client headers are adopted. These are still not implemented anywhere.
• I think we need to say exactly what fields in the EXIF need to be set. There is some discussion of that here Allow an image to indicate its own density and correct its intrinsic size whatwg/html#5574 - sounds like we need all four of PixelXDimension, PixelYDimension, ResolutionX, and ResolutionY, ... and ResolutionUnit.

[github-actions]

Preview URLs

• https://pr6798.content.dev.mdn.mozit.cloud/en-US/docs/Mozilla/Firefox/Releases/90
• https://pr6798.content.dev.mdn.mozit.cloud/en-US/docs/Glossary/Intrinsic_Size
• https://pr6798.content.dev.mdn.mozit.cloud/en-US/docs/Glossary/EXIF

Flaws

None! 🎉

External URLs

URL: /en-US/docs/Mozilla/Firefox/Releases/90
Title: Firefox 90 for developers
on GitHub

• https://github.com/eeeps/exif-intrinsic-sizing-explainer (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Glossary/Intrinsic_Size
Title: Intrinsic size
on GitHub

No new external URLs

---

URL: /en-US/docs/Glossary/EXIF
Title: EXIF
on GitHub

• https://en.wikipedia.org/wiki/Exif (1 time) (Note! This may be a new URL 👀)
• https://www.cipa.jp/std/std-sec_e.html (1 time) (Note! This may be a new URL 👀)

[Rumyra]

Amazing thank you @hamishwillee 🙌

I've started a Content Work project, so feel free to add any other tasks surrounding this work in there