So apparently this new rule/standard went into effect December 2016 causing this “Consider avoiding viewport values that prevent users from resizing documents” HTML5 validation error. To fix it make sure your:
Tag does not have the “maximum-scale=1.0” value, it’s that simple :)
*** UPDATE***
make sure you remove the following values:
- maximum-scale=1.0
- user-scalable=no
Source: https://github.com/validator/validator/commit/7cfc964d343cbd677beee32dad8f8e6ecab1210b
Thanks to Gaspard d’Hautefeuille for the update notice.