First of all thanks for all the answers. I was stuck in a different city because of heavy snow fall in south Germany and had no time for answering or responding, thank you for your patience.
@TW_Tones, I will try your plug-in immediately. Also thank you for explaining the problem why the browser spell check does not work, that is something I saw while searching for an answer but didn’t understood.
@Springer, yes, I would like to use CodeMirror and as TW_Tones wrote, its that the browser spell check does not work on HTML elements that are not some kind of text. (This is how I understood it now)
vilc, since I am just starting to use TW, so and wanted to take notes about code and put some examples in the tiddlers. But then as you say it may be not worth for me. I will try the plug-in first and then look if I will switch to the default one again.
pmario the reason I was posting is the last thing you posted here GitHub-TiddlyWiki-issue:6927 where you state that you found a way. I could not make it work for a long time, then after I got it work I read that turning on the spell check introduces some bugs and as a newbie to TW I dont want to change to much before I understand more.
Thank you all.
P.S. I am only allowed to mention 2 users as a new user.