Add support for @supports about css-validator

[marcoscaceres]

... or maybe change this to warning, instead of an error. It's preventing specs written using ReSpec from passing pub rules.

[palemieux]

See https://jigsaw.w3.org/css-validator/validator?uri=https://www.w3.org/TR/2017/WD-ttml-imsc1.0.1-20170323/

[ylafon]

Work started in the 'supports' branch, but not finished yet (as it require deeper changes than a regular @ rule)

[marcoscaceres]

@ylafon, great to hear. I'm wondering why @ supports is an error tho?

[devpaps]

I got a error today for @supports...

[raducoravu]

+1 for supporting "@supports". Like in:

        @supports (display: grid){
            body{
                display: grid;
                grid-template-rows: 100px 1fr 40px;
                gird-template-columns: 170px 1fr;
                grid-template-areas: 
                  "abc"
                  "def"
                  "hij";
            }
            header {
                grid-area: header;
            }
        }