Ban leading comma in URL for authors. Fixes #195
Ban 0w for authors. Fixes #196
Require unique descriptors in srcset for authors. Fixes #197
Simplify <source media> rule for validators. Fixes #199
Clarify no descriptors case
Make unique descriptors requirement tighter to disallow 1x and 1.0x
Merge pull request #204 from zcorpan/fix-195-196-197-199

Authoring conformance: fix #195 #196 #197 #199