Maybe it's just that everyone is too busy to shepherd the idea of commenting our HTML code in Canvas, but please don't be too busy not to vote for this idea. Currently in Canvas, all <!--comments--> get stripped out of code. I can't imagine why commenting wouldn't have been white-listed by Instructure; obviously the people wanting to do this know this is oftentimes the only thread to sanity when returning to code after a few months to enhance or troubleshoot.
Please read about this and vote up at: Enable Commenting in the HTML Editor
Also, look for separate Feature Upgrade Campaign announcement on: Grouped, Nested Modules, posted in ideas for voting as: Enable Commenting in the HTML Editor