Nodi is a promising web-based visual programming environment that, at first sight, reminds me of Grasshopper and can be used on a browser tab.
And, while writing these lines, I checked their home page.
Node is an online node-based geometry design tool and social development environment. […] You can create a model with a feeling of operation like Grasshopper and Dynamo.
Haven't looked in depth yet, but it seems to support some 2d and 3d geometry, math, exporting to formats like obj and dxf and stl, and much more.
I'll close with a Twitter quote from Masatatsu Nakamura.
It feels very intuitive to a Grasshopper user, but is built entirely on web tech.