Every browser opens it. The interesting question is what to do when you want the words out of it rather than the page.
An `.html` file is the page itself: the text, and the markup that says which part is a heading, a link, a table. It may also reference styles, images and scripts that live elsewhere, which is why a saved page sometimes opens looking like nothing at all.
Double-clicking opens it in your default browser on every desktop system. If it opens in an editor instead, right-click and choose Open with, then a browser.
On a phone, a file manager will usually hand it to the browser. If it refuses, emailing the file to yourself and opening the attachment normally works, because mail clients pass HTML to a web view.
A page saved with Save As, Web Page, complete keeps its styles and images in a folder beside the file. Move the file without the folder and the page loses everything except its text.
A page saved as a single file keeps everything inside it and opens the same anywhere. This is why an export worth keeping is a self-contained one.
Copying from the browser gives you the words and loses the structure: headings become ordinary lines, tables become runs of text. Converting the file to Markdown keeps the structure as something you can read and edit, which is usually what people actually wanted.