Resolve more W3C validation issues.
Resolve more W3C validation issues.

playi/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt,switchinnovations/pxt,Microsoft/pxt,playi/pxt,Microsoft/pxt,playi/pxt