Why are we receiving MSHTML warning when saving the Message Template?

Environment:

Message Templates
All versions

Issue:

When editing the Message Templates the following warning is received:

An MSHTML exception has occurred due to an issue with the source HTML.
As a result, the Design editor cannot render this code and will no longer be available for this document.

To restore Design editor access, you must alter your HTML to correct the exception, then save and reopen the document

Resolution:

The warning is being received when the code added to the Source of the Message Template contains proprietary Microsoft HTML code also known as Trident code.

In order to resolve this, the code needs to be removed and afterwards the warning will not be shown anymore.