said by NetFixer:Since you seem to have inside information (other than the information that is published on the test sites), how about enlightening the rest of us?
Its based on knowledge of browser security model. Standard browser programming environment does not have needed access to the operating system to cause a fragment to be transmitted nor does it have the ability to detect receipt of one (although this can be inferred the ipv6 test does not do it). Netalyzr does.
The PMTUD scheme used is also problematic. It works by sending spurts of data over TCP at a time followed by a delay to the users browser. When it DOES NOT work it can be an informative indication of a problem and is therefore useful.
However when it DOES work this does not necessarily mean PMTUD actually worked. It is very common these days for operating systems to be able to detect and work around systems foolishly electing to drop all ICMP using a variety of methods.
None of this works for UDP applications. If you have a UDP application such as a realtime video stream and there is no PMTUD you'll never know it unless the application has machinery to explicitly probe the end to end path.
Likewise if PMTUD does work but non-initial fragments are being dropped and you transmit a datagram > MTU it will cause breakage.
The IPv6 test site is simply not intended for this.