HTML and HTTP are open standards, but they're not the metaverse. The "metaverse" is supposed to be an open standard for interoperable 3D game worlds.

You could build a 3D game world inside of a browser, and people have been doing that since the 90s. We've had multiple different open and proprietary technologies that allow this: VRML, Java applets, 3D Groove SX/GX, Shockwave 3D, Flash Stage3D, WebGL, and WebGPU. However, most game worlds built this way are less like "websites but in 3D" and more like Facebook. Yes, Facebook uses HTML, but it's not part of "the web" - most of it's content requires a login and can't be scraped or searched by external sites.

An actual "metaverse built on the open web" would be possible; in the same way that comment sections, trackbacks, Gravatar, and so on made blogging into a shared space decades ago. You would need standards for a whole bunch of problems unique to 3D game worlds, including presence, chat, avatars, asset inventories, and so on. All of these would have to also be integrated with existing content-management systems (WordPress, Drupal, etc), which would also have to serve up a viewer application that actually handled our "web metaverse" thing (in the same way that browsers don't need to know about comment sections, they just need to know how to submit a form). That in and of itself has significant UX problems: maybe, you go on one particular metaverse site, and suddenly you have entirely different player movement from the last site you were on, because it loaded a different viewer.