The first occurence of the DSLReports speed test in the WayBack Machine is April 2000. January 2009. Indeed, the DSLReports is an oldie.
Sensitive to errors
The DSLReports speed test is a relative critical speed test in the sense that the test is aborted relatively easily.
As written at the homepage This test requires your browser's full attention for accurate results. Run the test when your network is quiet, and don't switch to other applications or change browser tabs during the test. A few Windows security and privacy software packages can block the test or slow the browser.
Error 7
Under linux I have tried the DSLReports speed test with different browsers (Google Chrome, Microsoft's Bing, Firefox and Vivaldi). In all cases I got "error 7" as a result. This despite the fact that the speed test had the browser's full attention.
Under Windows I got also error 7 when using the Firefox browser. The DSLReports speed test works fine under Windows with chromium based webbrowsers like Google Chrome, Microsoft's Bing and Vivaldi.
Losing focus: Error 24
When you switch tabs when running the speed test, you will get error 24. It is suggested that enabling web workers in the (advanced!) settings will solve this problem. That's indeed the case.
In summary: when this speed test works, it works fine, however it takes a long time (over a minute) to complete the test. Hence this speed test is not recommended.
In the speed test overview you find all findings regarding DSLReports's speed test.