Autoprefixer warnings about the partially supported align-items: start; CSS rule

[Snuffleupagus]

After PR #13479 landed, building the PDF.js library now produces warnings from the Autoprefixer package.
For example, with gulp generic the following is printed in the console:

[21:33:58] gulp-postcss: viewer.css
autoprefixer: C:\Users\Jonas\Git\pdf.js\viewer.css:383:3: start value has mixed support, consider using flex-start instead
autoprefixer: C:\Users\Jonas\Git\pdf.js\viewer.css:393:3: start value has mixed support, consider using flex-start instead

with those warnings originating at

• pdf.js/web/xfa_layer_builder.css

  Line 121 in 2a61110

  align-items: start;

• pdf.js/web/xfa_layer_builder.css

  Line 131 in 2a61110

  align-items: start;

According to MDN, please see https://developer.mozilla.org/en-US/docs/Web/CSS/align-items#browser_compatibility, only Firefox supports align-items: start; currently.

@calixteman Can we make the change suggested by Autoprefixer (in which case I'd happily submit a patch), or would that break the layout of XFA documents (in which case I'm guessing this is wontfix)?

Or, could it possibly make sense to at least amend the code with the following in those two cases?

align-items: flex-start;
align-items: start;

[calixteman]

I missed that... Could we add this css lint to gulp lint ?
Anyway, after reading the doc, I think replacing start by flex-start is fine.

[Snuffleupagus]

Could we add this css lint to gulp lint ?

Please note this warning only happens during building, and the gulp lint simply checks the source files as-is (since building as well would be quite a bit slower than it already is).
Furthermore, while there is a PostCSS plugin for treating warnings as errors it unfortunately (in my testing) doesn't seem to actually work with the gulp-versions of the PostCSS package that we use.

Given the above, I'm just going to fix the issue and defer the linting part for now (this is also the first time we've seen such a warning).

Edit: I've also tried the stylelint-no-unsupported-browser-features plugin, for our existing linting, but that doesn't appear to pick up this error unfortunately :-(