The HTML File Type, which uses the HTML Language parser, parses HTML elements along with client-side scripts elements. Client-side scripting blocks will appear using the syntax formatting appropriate for whatever scripting language is used. Symbols defined in the scripts are also displayed in the symbol window, and are saved in the symbol database.
Source Insight uses the script language specified in the SCRIPT element. You can specify the default scripting language to use in HTML in the Options > Preferences: Languages dialog box. Click on the Special button and select the default language where it says "Default script language".