I this blog post I will explain how to add your own custom (client-side) validation to a SharePoint 2013 Page Layout.
As you already might know a page layout if often associated with a content type within SharePoint. Every page layout is associated with a content type, typically one of the content types in the Page Layout group. For example, the Article Page content type that is associated with the Article Page page layout, both of which are included in a publishing site. A content type is made of site columns, which together define a schema of allowed data types.