Isolate container div from website’s styles without iframe

I have a simple textarea and a preview modal which shows me how the content of the textarea looks like in an email (so basically loads and shows the content as HTML).

Now, this preview is affected by my website’s styles and I want to isolate it, so that it’s as plain and exact as it would show when sent as email. I don’t want to use iframe as it’s not responsive, are there better ways to do it?

I’m using VueJS, but ideally I would like a solution that doesn’t require installing a plugin just for this. The solution I’m looking for is either in CSS, or vanilla javascript, or built-in Vue functionality