Today, my colleague Freek asked for help embedding the webview of an email campaign in an iframe. He needed it in an iframe because embedding the HTML directly caused layout issues because the website's CSS clashed with it.
After setting up the iframe, we needed to find a way to dynamically resize it based on its contents to avoid double scrollbars on the page. While possible, it required some icky scripting.
I took a step back. The problem at hand was that the CSS needed to be scoped somehow. While iframes were the only solution for a long time, these days we have the shadow DOM.