[css-images-4][css-shapes-1] Allow mixing `<radial-extent>` and `<length-percentage>` in `<radial-size>`

[cdoublev]

Two <radial-extent> keywords can now be specified for ellipse() or an elliptical radial-gradient(), thanks to 351600a.

For example, radial-gradient(farthest-corner closest-side, yellow, magenta) is now valid.

This was proposed in #824 but @astearns also suggested a syntax that allows mixing <radial-extent> and <length-percentage>.

For example, radial-gradient(farthest-corner 100%, yellow, magenta) would be valid.

But the current syntax does not allow it:

<radial-size> = <radial-extent>{1,2} | <length-percentage [0,∞]>{1,2}

Is this an oversight or is there a reason to not use [ <radial-extent> | <length-percentage [0,∞]> ]{1,2}?