Rocket Validator automatically checks your pages on the W3C Validator.
HTML issues tagged as mso.
An invalid CSS property is being used. Properties starting with
mso- are commonly defined by Microsoft products like Office and Outlook.
These properties, like
mso-margin-top-alt and others starting with
mso- are not standard CSS properties.
When was the last time you validated your whole site?
Keep your sites healthy checking for A11Y/HTML issues on an automated schedule.