Add SVG opacity properties as exceptions to alpha-value-notation

[Mouvedia]

What steps are needed to reproduce the bug?

use one of these SVG properties:

• flood-opacity
• fill-opacity (also in SVG2)
• stroke-opacity (also in SVG2)
• stop-opacity (in SVG2)

ref w3c/csswg-drafts#3342

What did you expect to happen?

to be exceptions as well

What actually happened?

ref #210
see also https://test.csswg.org/harness/details/css-color-3_dev/opacity-computed/

[jeddy3]

@Mouvedia Thanks for the issue and for providing all the references.

The rule currently only checks the opacity and shape-image-threshold. We can add the properties above to the rule and then as exceptions here in the config.

Do you want to open a false negatives issue for the properties above in Stylelint?

[Mouvedia]

Do you want to open a false negatives issue for the properties above in Stylelint?

If both releases of stylelint-config-standard and stylelint are synchronized, yes.
Else Id prefer if the exceptions were added beforehand—even if they are inactive for a short while.

[jeddy3]

If both releases of stylelint-config-standard and stylelint are synchronized, yes.

Yes, they're synced. We do new releases of the configs, if we make any changes to Stylelint that impact either of them.

[Mouvedia]

Yes, they're synced.

Alright then. Ill either open an issue or a PR.

[Mouvedia]

Do we really want to have 5 exceptions out of 6 properties targetted?
i.e. it only prefers percentage for shape-image-threshold

		'alpha-value-notation': [
			'percentage',
			{
				exceptProperties: [
                	'opacity', 
                	'fill-opacity',
                	'flood-opacity',
                	'stop-opacity',
                	'stroke-opacity',
               ],
			},
		],

I think the saner option should be to just pick number as the default.
When it's 5 out of 6 it's not really exceptions anymore…
@jeddy3 @ntwb

[jeddy3]

We can remove each exception when it has fuller browser support. Percentages feel more natural, especially in colours, e.g. hsl(0deg 100% 50% / 25%), which I assume is why they've often been used in graphics apps and been added to the language.