One fun thing I did once is use the custom elements API to make a custom tag <element-template>. When the page sees <element-template tag=what-name></element-template>, it looks inside, finds any <template></template> <script></script> and <style></style> tags, and builds a new custom element with that tag name and all the powers. Cool but not incredibly simple.

Another amazing trick is to put a mutation observer on the page to detect <template> tags as they're created. (template tags parse the HTML within but don't actually create it on the page). Once the observer finds a template tag it can upgrade them to have new powers, like replacing the contents of other tags. There are ways to tame web components and make them very easy to use.