1. Create a HTML5 project Prj1
2. Create a Java EE 7 web project Prj2 (this will now create index.html)

=> If index.html from Prj1 is focused, the CSS Styles window has 2 tabs: Selection and Document. However, if index.html from Prj2 is focused, it has only 1 tab. 
Next, the contents of this tab (pane) is determined by the recently selected tab in index.html from Prj1 (so if you select Document when Prj1's index.html is active and then switch to Prj2's index.html, it will show the Document content, and same if Selection was selected).
Finally, if Selection was selected, then there is the button "Run index.html", but this is enabled for Prj1's index.html and disabled for Prj2's index.html.