Writing HTML, and therefor javascript, in parameters is unsafe. Also it is very likely for someone to break the HTML of the page. If you want something like that you must do it elsewhere, eg with something like bbcode or find the replace.

I just tested it, although the reason is obvious as I already said.
I included an image the way you wrote and saved normally. See the screenshots.
The reason is 100% the one I wrote. The session token is been overwritten and token gets altered. If you haven't opened a secondary window then you must search for the reason in your browser, in an anti-virus or similar. Something might interfere between the site and the browser.

The reason is obvious as you have a very specific error code: E403-CCON-0013
This is from component content, controller file aarticle.php line 1036

if (($token == '') || ($sess_token == '') || ($sess_token != $token)) {
   exitPage::make('403', 'CCON-0013', $eLang->get('REQDROPPEDSEC'));
}

This issue happens with every Elxis installation I have. How do you open simultaneously multiple edit pages? I don't even know how to do that.

Wrong topic. Check here

No, why to do such a thing? There are workarounds on this (eg use replacements like BBcode bold, etc), but as you develop your own template you don't need such parameters. Apply the style in your template directly.