Thanks for the reply!
I know iFrames are not a clean solution, that's why I'm asking for alternatives.
But you can do almost magical things with it^^ and it was just my first approach cause I did not find any other standard object type that could do the job.
The hint with the plugin let me to this one https://www.construct.net/en/make-games/addons/190/html-element.
It does the job by adding a new object "HTML" to Construct. But you have to assign the HTML Tag in the editor. I would prefere simply writing some code as text which is much faster.
There seems to be a workaround.
Every HTML object has a 'Text like HTML' checkbox. This allows you to put HTML in the text editor. It might do the job, but I have to test it.
I've expected a way like in Phaser.io or PlayCanvas. There you can simply load and render html files with CSS and JavaScript.