Is there a reason why native and HTML5 games are strictly separated so that I have to choose the platform already when I create a new project?
I was naughty and created two simple projects one for HTML5 and one native, then I exchanged the platform strings in the “Project.gdg” files and sudently the games could be compiled for the other platform.
Wouldn’t it be better to add a flag to each automatism and stuff that marks it as available for HTML5/native/both. That would enable us to create a general project and decide about the platform later while developing the game. If I only use those features that are available on both platforms I could export the game to HTML5 or native binaries and if I for example use stuff that is only marked for native the HTML5 export option would be grayed out.
It’s about extensions from what i know, and it’s all related to sfml (for example particule system). But yes i agree it would be nice to have flag, work on a common three then make 2 different exports.
You can de/activate platforms in the extensions window, right click in the platform and then “Do not/Use this platform”. If you activate both platforms, you can select in wich platform preview with the little arrow under the Preview button 
By the way, you hav to re-open your scene after change the platform targets to update changes 
Yes, this is about extensions, but not only related to SFML. For example multitouch is only available for HTML5 games, and not-graphic related extensions as Pathfinding were only available in native games for a while
And again I learned something new. ^^
Thanks guys.