[css-page-floats-3] Add missing CSS2 'both' keyword to 'clear' grammar

[cdoublev]

Fixes #7350.

There has been no update to the spec since 1 year but I would hope that at least both, which is defined in CSS 2, could be re-included in the value definition of clear, for backward-compatibility, noting that:

1. both-inline and both-block are also defined in prose in CSS Page Floats 3 but are missing in the value definition of clear
2. both is defined in CSS Page Floats 3 as equivalent to both-inline, which is defined as equivalent to inline-start and inline-end

[dbaron]

I don't think css-page-floats-3 should be presenting itself as the primary definition of the float and clear properties. This is a set of additions to those properties. The primary definitions should be in a different level 3 module: maybe css-box-3, or maybe a module for block layout.

[cdoublev]

Oh ok, thanks. Should I create a new issue or should I change the title of #7350 by replacing [css-page-floats-3] with [css-box-3]?