[css-content] Provide a way to generate copyable content

[jyasskin]

https://w3c.github.io/csswg-drafts/css-content/#content-property doesn't define whether the content it generates is copied when a selection containing it is copied, and Chrome 106 and Firefox 102 don't copy the generated content. For quotes, it's important that the generated content gets copied, although there are probably other use cases where it's important that it not get copied. So I suspect there should be a new property to generate copyable content, rather than changing the behavior of the content: property.

See also #6222 for the question of searching for generated content.

[Loirooriol]

See https://w3c.github.io/csswg-drafts/css-ui/#propdef-user-select

The UA may apply this property to the ::before and ::after pseudo-elements. If it does, the auto value maps to a used value of none on these pseudos, but other values can be specified. This preserves the legacy behavior of generated content not being selectable or copyable, which is appropriate when these pseudos are used for decorative purposes. However, this property allows them to become selectable and copyable, as the user would expect in cases where they are used to generate parts of the content, such as the issue numbers in this document.

[jyasskin]

user-select would be a plausible way to implement this, although it doesn't have a great name for it. It would "just" need to drop the "optionally" in its "Applies to" element.