I like web components, the simpler the better, so I like what I see but having a strong OCD about syntax I just want to propose the use of <Button ... instead of <elena-button ...

It's just a matter of using a regex [1] and making your syntax more palatable

Regardless, kudos for the release

* const componentRegex = /<%[ ]+([A-Z][a-zA-Z0-9])([^%]?)%>/g;

HTML is case insensitive, so <Button> is the same as <button>, and custom element names must have a hyphen.

Your component regex represents a JSX limitation, but web components do not need React. Plus, if you capitalize the first letter, JSX would treat it as a React component rather than a web component (i.e. custom native HTML elements)