Where and how would be the best way to save game in a webplayer game? I know how to in a standard exe build but not webplayer version.
You could use a SQL database on your server. In the wiki there is an example (for saving highscores) with SQL, PHP and JS code: Server Side Highscores
You can use PlayerPrefs, but the PlayerPrefs filesize on WebPlayer is limited to 1MB per URL, per user, per computer.
Usually when setting up a Game Save, I have to think outside of the box to get the most of the game save.