In scripts, you can use common HTML5+JS. Chromium is used for rendering, so any HTML5/JS standard supported by Chromium (currently version 64) can be used.
MM5 contains framework for auto-tagging (see bin\helpers\autoTagFramework.js, there is some description), so there is probably no need to create own UI for another tagger. Currently MM5 contains 3 taggers (implemented in helpers/searchMusicbrainz.js, helpers/searchTVDB.js, helpers/searchIMDb.js), you can register another in the same way.

Thank you for trying to help me. I don't know where to start writing an user Interface. JS? Chromium?

I try to convert Discogs Tagger to MMW 5. So i need an UI like in the Discogs Tagger in MMW 4.

Sorry if this question is a dumb one...

Hi, could be more specific, what do you need? I am not sure. Currently e.g. lastFM script (in Scripts/lastFM) adds own panel to options dialog by using dlgOptions_add.js. Maybe try to describe some sample, which would help you, and we can make it.

Hi,

what is the best way to write a Chromium UI with objects that can be handled by the script?
With an example?

Thank's for any help.