[meta] Remove support for Internet Explorer

Yay! I was coming here to create this issue!

Here's a quick braindump:

• Removing polyfills (this should be the easiest)
• Yanking Explorer from https://git.drupalcode.org/project/drupal/-/blob/10.0.x/core/package.jso... and then recompiling CSS
• Removing the unnecessary compilation of CSS Variables (possibly setting postcssCustomProperties to false at https://git.drupalcode.org/project/drupal/-/blob/10.0.x/core/scripts/css...)
• Remove unnecessary @import statements from each CSS file within Olivero and Claro (and other places that use *.pcss.css files.

If I have time later this week, i'll come back and edit the issue summary if anyone hasn't gotten to it yet.

Added a list of polyfills

Adding CSS tasks

Listing out some subsystems.

IMO, all JS polyfill can be removed together in a single thread. This way you can perform batch SEARCH to find out what LEFT.

AFAIK, it should be a good pattern to SEARCH & REPLACE(REMOVE). If not, we really need to revisit how bad we've done before and improve/avoid it for the next level :)

Just getting this into the right place in the D10 issue tree.

Oops, some of the children can go in before alpha1, but not necessarily all of them.

Updating polyfill issue to #3238501: Remove empty and deprecated IE11 polyfill library stub entries in 11.0.0

I've set these as child issues:
#3101620: Remove IE conditional comments support in Drupal 10
#3214211: Stop setting X-UA-Compatible header

Also this? #3256549: Deprecate core/drupal.date asset library in 9.4.0

#3254202: Remove IE11 Support from Claro is close although it could use some help to get it over the line. A couple of Olivero issues still to go too.

The IS mentions settings tray and media, but there are no issues for those, should there be?

I think for Settings Tray, the issue was #3291797: Refactor Drupal 10 settings tray / off-canvas to use modern CSS.

Changing this to a beta 'should have' since most of these issues aren't actually beta blocking. Anything that is/was beta blocking should already have been surfaced separately.

Moving this down to major since it's not really time sensitive, we just need to finish off the child issues, some of which are 11.x only like dependency removals.

$ grep -ri IE11 --exclude-dir="node_modules" --exclude-dir="assets" core
core/themes/claro/js/tableselect.js:        // is preferable, but not supported by IE11.
core/themes/claro/js/tableselect.js:          // Determine add/remove with ternary since IE11 does not support the

Made an issue for this, #3540893: Remove IE11 Support from Claro

The last child issue is fixed. And there are no more instances of the text 'IE11' in core. However, there are still outstanding tasks in the issue summary.

Are there any more instances of IE11 support that need to be removed? Or can this be closed now?

Regarding this task:

Determine which (if any) PostCSS plugins can be removed

The postcss-preset-env plugin says this in its docs:

PostCSS Preset Env lets you convert modern CSS into something most browsers can understand, determining the polyfills you need based on your targeted browsers or runtime environments.

This sounds like the relevant plugin configuration to look at in core/scripts/css/compile.js. I don't know anything about PostCSS. I may be able to figure out which features were enabled for IE11 support, but due to my inexperience I wouldn't know without study.

Here's the configuration from compile.js:

      postcssPresetEnv({
        stage: 1,
        preserve: false,
        autoprefixer: {
          cascade: false,
          grid: 'no-autoplace',
        },
        features: {
          'blank-pseudo-class': false,
          'focus-visible-pseudo-class': false,
          'focus-within-pseudo-class': false,
          'has-pseudo-class': false,
          'image-set-function': false,
          'prefers-color-scheme-query': false,
          'content-alt-text': false,
        }
      }),

The only mention of IE11 in the original issue that added postcss-preset-env was in regard to the autoprefixer setting.

From that issue:

We also need to turn on CSS Grid support for IE11 (Olivero uses CSS Grid extensively). CSS Grid is disabled by default in Autoprefixer (see https://github.com/postcss/autoprefixer/blob/master/README.md#options), and can be turned on by setting either grid: 'autoplace' or grid: 'no-autoplace'. The autoplacement feature didn't work for us, so have the latter option set.

I think we might want a separate issue to revisit whether we need postcss at all these days.

OK some of that sounds worth having so not worth trying to get rid of it entirely. Although shifting what we can to linting and checking if we're using the right supported browser configuration is probably worth it?

When we originally dropped IE6 support we made a decision not to actively remove XSS protection in PHP etc., but that was over a decade ago, so trying to clear a lot of that out now feels overdue.