I'm not sure about React's capability with this, but Vue can very easily run with no build stage. You can load Vue from a CDN and use a couple lines of JS to point it at a DOM element in your app. Then you can write a plain JS Vue object (what goes inside the <script> tags in a .vue file), and all this logic will apply inside the element you pointed it at. You can also write components, although that gets a little clunky without a build stage.
This has allowed me to do things like create Vue-based tools in a locked-down corporate environment where I was not able to install Nodejs on my machine.
This has allowed me to do things like create Vue-based tools in a locked-down corporate environment where I was not able to install Nodejs on my machine.