Bug: Link here
Commit: Link here

At the last checkpoint for this bug, my code had been reviewed by John Buckley and he had suggested some changes.

Here’s the update:

/* IE9+ Compatibility mode fix  - Bug 1606*/  
if (document.documentMode >= 9 && !document.doctype) {
  throw("DocType directive is missing. The recommended DocType in IE 9 is the HTML 5 DocType: <!DOCTYPE html>");

Tested it on IE9 to make sure it works, results below:

PJS sketch in IE9 without any DoctypePJS sketch in IE9 with HTML5 Doctype

Comment now!