Interesting. I'm familiar with the "This taks is too nebulous-what am I even doing?" but it isn't a source of procrastination for me exactly. Usually it's a cause of spending well over 25 minutes stuck in thought loops trying to figure out what to do, and what I actually need to do is talk it through with somebody or at least think out loud.

"A few minutes of browsing won't matter" matches procrastination for me, but even your short comment suggests a different context for the quoted phrase than I experience. For me "A few minutes of browsing won't matter," isn't part of the problem, it's a symptom of it, or even an instinctive attempt to solve it. You are implying that thinking such things causes you to browse instead of starting the task. For me I say that in order to guilt myself into not starting a longer-more immersive fun activity, thus giving up on the task.

For me procrastination consists of cycles of (look at website-try to start task-attempt fails-stare at nothing until boredom requires me to seek a stimulus-seek stimulus in something like browsing where the attention chunks are smaller so I'll be able to try again sooner-look at website...)

Is this not the usual phenomenon? To clarify "attempt fails", what failing looks like is this, my mind seeks to give the command to do the first step of the task, but afterward I notice my muscles have not moved.