The timebox has to be enforced on the company side, meaning that you have e.g. 2h to submit your answer once you've opened their link (which you should be free to open at any time, so that you're sure you have the time allotted).
I think the <textarea> problem can be solved if the interviewer gives enough information so that the interviewee can have their IDE working correctly by the time they get started.
The pressure problem can be solved by giving more time than what would be expected in a work setting. If you expect a task to be completed in 1h, just give 2h. Of course that implies that you don't give a task which would take an expected 6h to complete.
Of course these are things to consider from the company side, if they think they want to hire people who can program under high pressure, they should probably keep that as part of the interview, for both the company's and interviewee's sake.
