Add alpha-value-notation

[jeddy3]

Which issue, if any, is this issue related to?

Closes #4720

Is there anything in the PR that needs further explanation?

I added an exceptProperties option as support for percentage alpha-values for opacity was added in a later spec, it isn't supported in some browsers like Safari, and I don't think postcss-preset-env transpiles it yet.

When this rule and the other two new rules are in, I'll create a follow-up issue to refactor some of the shared functions into utils, e.g. getValue and setValue.

[m-allanson]

Also looks great 👍

[mathiasbynens]

I added an exceptProperties option as support for percentage alpha-values for opacity was added in a later spec, it isn't supported in some browsers like Safari, and I don't think postcss-preset-env transpiles it yet.

Note that the latest stable Safari version (13.1) supports opacity: 50% according to CSS.supports('opacity', '50%');. I don't know what the oldest Safari version is that supports it though. Also, Opera is Chromium-based, so it likely actually has support for it too. TL;DR Take the MDN support data with a grain of salt. (I think exceptProperties is a good idea regardless 👍)

[jeddy3]

@mathiasbynens Thank you for the review. I'll release the new rules now.

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Changelog entry:

• Added: alpha-value-notation rule (#4770).

[pascalduez]

As a possible ref, we just hit NMFR/optimize-css-assets-webpack-plugin#118 after upgrading to stylelint 14, stylelint-config-standard 23.

TL;DR: cssnano badly transforms opacity percentage values. Should be fixed by now, but you know stuck transitive dependencies, lacking time to upgrade tooling like Webpack etc.

[alexander-akait]

Please migrate on https://github.com/webpack-contrib/css-minimizer-webpack-plugin (new cssnano version), if it will not help, please open an issue in cssnano