Using cm/mm on the CSS of a web app that replicates paper interaction is a good practice?

I am building a web application that interact with documents for later printing. In some points is similar to Google Docs. I am considering using cm/mm on the CSS of my documents pages because it will help me on the document generation. Example:

// A4 size
.page {
   width: 210mm;
   height: 297mm;
   margin: 2cm 5cm; 
}

<div class="page">
  ...
</div>

What are the main issues of following this approach?