2.1.74 [W3C-HTML51] Section 4.11.5.2 Processing model

V0403: The contextmenu attribute is not supported

The specification states:

 4.11.5.1. Declaring a context menu
  
     The contextmenu attribute gives the element’s context menu. ...
  
 4.11.5.2. Processing model
  
     Each element has an assigned context menu, which can be null. If an element A has a 
     contextmenu attribute, ...

EdgeHTML Mode

The contextmenu attribute is not supported.