Lightweight HTML viewer I'd like to write an HTML app for my own use that works offline. I'll edit my source code in Sublime Text 3. I don't have a lightweight app to render and show it.

I do know I can open HTML files in a Web browser, but opening an offline small HTML app in a Web browser such as Firefox seems too much. I'll consider using Safari too, but is there an even lighter HTML viewer out there?
You have not mentioned a platform explicitly in the question, but from the context it appears like you are looking for a macOS app.

For the scenario you described, I'd advise you to use the Quick Look feature built into macOS. It lets you quickly preview a file's content in Finder. The default installation of macOS include support for common file types such as HTML (along with a host of other common file types such as PNG, JPG, MP3, PDF etc.)

To Quick Look your HTML file, simply click to select your file in a Finder window and press the Spacebar key. To dismiss the Quick Look preview, press the Spacebar key again or press the Esc key.

Quick Looking a HTML file will render the HTML preview of file instead of showing the HTML source.