When you start editing a page, the editor is initially in the Visual mode, so every time you have to switch it to the Source mode if you prefer html-level editing as usually programmers do. It is annoying when you have a lot of things to do during the day, especially when there is a bunch of languages every page is translated to.
I haven't found any option in the CMS and no solution in Google. Actually I found myself the way to change this, so I post this question in order to instantly answer it. Maybe this will help somebody.