What this measures
Shows navigator.connection when it exists: effective type, downlink hint, round-trip hint, and save-data. The downlink field is a browser hint, not a speed test.
How to read it
Separate the calculators from the live checks. A calculator never opens a connection. A live check only talks to this website, and the byte count is part of the result.
What it cannot do
There is no upload endpoint and no packet-loss stream. A fast download of a small file can finish before the link is full. The path is only as far as this website.
Questions
Is the download test my ISP speed?
No. It times a file from this website. Distance, the sample size, and the host all sit in the number.
Why is there no upload reading?
A static site cannot accept an upload. The calculator only does the arithmetic you type.
Where is packet loss?
It is not shown, on purpose. A page cannot see dropped packets without a server that counts them.