Geodatadownloader[1][2] solves the problem of "how do I download data from this ArcGIS rest endpoint?" It operates completely in the browser, besides the CDN for the javascript, as well as the ArcGIS online map used to visualize the data you're about to download.
Long term, I'd love to add support for download of multiple file formats. I believe I'll be able to use an emscripten port of GDAL (specifically ogr2ogr) to do this.
[1] https://geodatadownloader.com
[2] https://github.com/mchaynes/geodatadownloader
