What's the format of official "online" HSK tests?
I took the computer version HSK4 test.
Audio was only played once as standard for levels 4-6.
You get some small headphones which did cut out the exam room background noise a bit. (If all the levels are being held in the same room then there can be chatter as the lower level test finish earlier.)

Pros:


Don't have to write characters (this is probably only a pro for levels 4-6 because the testable characters in HSK3 are usually very simple, 米， 大， 小， etc.).
Can mark reading questions you're not sure on and quickly click back to them when you have some time at the end. (You might be able to do this on paper anyway.)
Also if you can't remember the pinyin for a character but you know what it means this can be frustrating. But you can cycle through all of your guesses to try and find it in the entry system.


Cons:


Reading font was not the one I was used to and took a little longer than normal (for all parts).
You can not skip ahead on the listening part and read all of the questions if you answer one quickly, you must wait. This is especially annoying for the two part audio questions, as you have to remember the whole audio snippet and wait for the second question to come.
The writing entry system for typed pinyin was not the one I was used to and would struggle to automatically generate words.
There is a character limit on the written test which is not mentioned on the mocks for written exercises.


Extra:


The test centre I was in had a technical failure and the tests wouldn't start. When they did start the audio had already played for the first 2 questions and wouldn't replay, which wasted time and stressed me out for the 3rd question etc. The staff said they couldn't do anything to adjust the marks or retest the questions because it's a centralised test.