docs: clarify usage of very low opacity on some inputs

[jawinn]

Description

Adds some comments to clarify existing CSS that uses a low opacity (0.0001 and 0.000001) for visually hidden inputs. And explains why they are hidden this way with sizing and positioning for touch users, instead of using a screenreader only (.sr-only) technique.

Reference info:

• Inclusively Hiding & Styling Checkboxes and Radio Buttons: Hiding the checkboxes inclusively (2020) - Why this technique appears to be used and why a visually-hidden or sr-only class would not be an improvement over the current way that these inputs are hidden.
• Inclusive Considerations When Restyling Form Controls (2018) - Source code that referenced the ChromeVox issue as being the reason that a very small but not zero opacity is used. This issue is also mentioned in a few old tangential GitHub issues and on StackOverflow. I wasn't able to find any other reasoning for why this technique would be used, as opacity: 0 elements are included in the accessibility tree.

At some point in the future we may want to look at dropping the very low opacity in favor of opacity: 0. I cannot find any official change log available that references exactly when the ChromeVox issue was fixed.

CSS-814

How and where has this been tested?

Please tag yourself on the tests you've marked complete to confirm the tests have been run by someone other than the author.

Validation steps

• Validate that explanation and wording are correct

To-do list

• I have read the contribution guidelines.
• If my change impacts documentation, I have updated the documentation accordingly.
• ✨ This pull request is ready to merge. ✨

[changeset-bot]

⚠️ No Changeset found

Latest commit: bc29915

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[github-actions]

🚀 Deployed on https://pr-2963--spectrum-css.netlify.app

[github-actions]

File metrics

Summary

Total size: 4.63 MB^*

🎉 No changes detected in any packages

* Size determined by adding together the size of the main file for all packages in the library.
* Results are not gzipped or minified.
* An ASCII character in UTF-8 is 8 bits or 1 byte.

[pfulton]

Thanks for leaving these comments! This will come in handy in the future when we look back to refactor these :-)

[marissahuysentruyt]

Looks good- thanks for researching this and leaving some explanations!