Not to support or attack the rationale behind the css or html standards but these have exact real world SI unit meanings:

   CSS  |                    | Exact Size | Exact Size    
   Unit | Name               | (Inches)   | (Millimeters) 
  --------------------------------------------------------
   cm   | Centimeter         | 50/127     | 10            
   mm   | Millimeter         | 5/127      | 1             
   Q    | Quarter-millimeter | 5/508      | 1/4           
   in   | Inch               | 1          | 127/5         
   pc   | Pica               | 1/6        | 127/30        
   pt   | Point              | 1/72       | 127/360       
   px   | Pixel              | 1/96       | 127/480                 

https://en.wikipedia.org/wiki/Millimetre

https://www.w3.org/TR/css-values-3/#absolute-lengths