Using this API: https://developer.mozilla.org/en-US/docs/Web/API/Network_Information_API
You can run $ navigator.connection in a browser console to receive your different values regarding your network connection.
However the downlink attribute is a max of 10 (aka 10Mbps). Why is it capped here? Doesn't really help me since I need more info since I am deciding whether a client can handle HD video that may very well require over 10Mbps, thanks.
I found the answer in the comments to this answer: https://stackoverflow.com/a/47511842/3973137
Turns out Chrome caps it at 10 Mbps to prevent fingerprinting