is there any possibility to view equations in the HTML-Editor like they were displayed in preview?
I assume by "equations" you mean LaTeX markup. If so, the answer is No. The HTML must be sent to the server
in order to have the LaTeX rendered by the TeX filter. Depending on your context
, however, you may be able to submit the page, view it, and then edit it. For example, in a Forum, after you submit a post, you have 30 minutes to change it. It would be nice to have such a feature in Quiz (save my quiz and review what I've submitted exactly as the teacher will see it), but there is no such feature as far as I can tell (you can review your answers, but they're not rendered).how is it possible to edit existing equations with the DragMath-Editor
It would a nice feature, but it is theoretically impossible. There is no function from 1-D notation to 2-D notation (a given LaTeX string may have more than one interpretation). It is a theoretical limitation, not a limitation of DragMath. Chris Sangwin explained it very nicely in a forum post which I cannot find.