Support CSS color module level 4

[sander-deryckere]

When using a color value without commas (like style="color: hsl(180 100% 50%);"), the resulting color is black. While color values with a comma (like style="color: hsl(180, 100%, 50%);") work as expected.

Both should give the same results: https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/hsl

[liZe]

Hi!

That’s a feature introduced in CSS color module 4.

[liZe]

It will require some work in Kozea/tinycss2#47.

[liZe]

If anyone is interested in implementing this feature, I’ll be happy to help! That’s a fun feature to learn more about web specifications, and it shouldn’t be too difficult to get nice results with a small amount of code lines.

[Thomasb81]

Hello

Apparently to fix this issue following function need to be fix:
https://github.com/Kozea/tinycss2/blob/291c1429e7de23d839c8bc187908b967013db477/tinycss2/color3.py#L126
in order to support 180, 100%, 50% or 180 100% 50%

But I understand also that hsl(180 100% 50%) is new syntax introduce by css-color-4 which is not yet approved W3C standard.
So if this fix is implemented is there other feature of this draft that need to be anticipate ?

[liZe]

So if this fix is implemented is there other feature of this draft that need to be anticipate ?

Actually, we should keep the color3 module and write another color4 module in tinycss2, in order to let users choose the version they want to support. We’ll then be able to use color4 instead of color3 in WeasyPrint.

It’s safe to implement almost everything from this specification. Even if it’s only a working draft, some of its features are already implemented in other browsers, it shouldn’t change much in the future. Libraries using tinycss2 will be free to use the version they want.

[Thomasb81]

color4 seems far more complex than color3.

In particular there is support for Device-independent colors and Predefined Color Space

Where the purpose if I understand it well is to have high fidelity color rending on device : monitor in case you read you document on a calibrated screen or on paper-ink-printer to preserve author intent.

Can such application be supported by PDF and WeasyPrint ? I may be wrong but I guess today only RGBA is supported and everything is converted to RGBA. With Color4 I understand that the specified color space should end-up in the pdf document.
Implementing everything on this topic required sharp knowledge and very few user may use them ...

[liZe]

color4 seems far more complex than color3.

Yes, it is.

I may be wrong but I guess today only RGBA is supported and everything is converted to RGBA.

Unfortunately, you’re wrong. CMYK support, for example, would be very useful for WeasyPrint.

In particular there is support for Device-independent colors and Predefined Color Space […] Can such application be supported by PDF and WeasyPrint?

There are three different steps.

The first step is to handle the syntax in tinycss2.color4. This part is necessary before handling what we can/want to handle in WeasyPrint. For this step, there’s no need to assume whether device-independent colors or predefined color spaces can/will be supported by PDF and WeasyPrint.

The second part is to use tinycss2.color4 instead of tinycss2.color3 in WeasyPrint. It’s the first step giving the possibility to handle new syntaxes (ie. fixing this bug).

The third part is to include new features, in the future. We’ll answer these questions then.(Spoiler alert: they are useful, for example for PDF/A support that can require default ICC profiles to be embedded. That’s just an example, CMYK being another obvious one.)

Implementing everything on this topic required sharp knowledge

Do you imply that implementing countless CSS specifications, HTML, PDF generation or SVG rendering doesn’t require sharp knowledge 😁? We can learn, as we always do.

More seriously, we don’t want to handle everything in WeasyPrint right now. We only need the two first steps, and they are the easiest ones.

and very few user may use them

I wouldn’t assume that CMYK is only used by "very few users" in the print industry 😁. And among the 7 million downloads of tinycss2 last month, it’s probably possible to find a few users that would benefit from a clean support of the level 4 syntax.