The AMP specs do not allow !important declarations in custom styles. We have a few in the example subtheme's CSS, which causes validation errors.
Remove those !important declarations.
Most likely impact is on the skip link styles. Check if that still works.
Comments
Comment #3
rainbowarrayFixed for 8.x, tested and skip link still works. Moving on to 7.x.
Comment #5
rainbowarrayFixed this for 7.x too. Again, tested and skip link still works.