Steps:

• In Windows settings, set html files to open with Notepad (or something else) by default.
• From the Unity help menu, click any of the documentation menu items.

Result:

• The HTML file corresponding with the menu item is opened in the default application (for instance notepad).

Expected:

• Detect default browser and launch the html document in that.

Workaround:

• Associate HTML files with a browser in stead. NOTE: This will potentially break user workflow for HTML file editing.