There is a minor style bug in SharePoint 2013 (including SharePoint Online). The error message on a required DateTime field is not displayed in a manner consistent with other control validation errors. No it’s not just you, and no it’s not due to some conflicting CSS – it is a SharePoint bug.
Specifically I am referring to the page layout edit experience. A user fails to provide a value for a required
DateTimeField control and the validation message is shown in the default text colour – ‘You must specify a value for this required field.’
For all other validation messages the SharePoint controls add the
ms-formvalidation which sets a CSS rule to set the red colour. This is the only rule which the
ms-formvalidation class sets and as such this is the only rule that should be applied to fix the issue.
I use the following CSS selector to resolve this issue: