A few months ago Camlaman wrote an example to help write extensions using html dialog. ([Example]An example to help you how to write SketchUp Ruby code with HtmlDialog) Topic now closed.
Well it did help me mainly because it was reduce to most simple expression.
Of course this is not the way a rb file should be written (it was not intended to be such).
So as an exercice, I wrote it as an extension. And to further help, following his intention, I post the result.
The referred html file, box.html, must be in a folder named htmlF, on same level as the rb file.
box.html.zip (1.1 KB)
If you put these in the plugins folder, the script will run at SU launch.
Now my next quest is html localisation.