Is there a markerless AR library that can run on any mobile web browser? Something that does not require a standalone app like Google Article, and can be run on both Android and iOS.
For that to happen, the mobile browsers would have to be able to support some web-based AR framework and properly interface with the hardware components such as camera, gyros, etc.
There are a few web-based AR projects out there, the most popular is currently AR.js (loosely branded as WebAR) which is what Google Article is currently based on. However, they are limited to experimental browsers or special "WebAR" applications with the correct hooks.
An alternative is to consider using cross-platform AR frameworks, such as Viro React which enables mobile cross-platform AR/VR development in React Native (Javascript-based). However, you will need to build your experience into an App in order to deploy it.
However, with time, WebVR and WebAR should eventually be supported by the various browsers and their vendors.