You mean that some wikitext widgets don’t allow internal html elements? Could you give an example? Is there even a list or some rule of thumb to know which? Or do you mean something else?
The above is theoretically faster in situations where the value of a variable changes and the contents of the $let widget need to re-rendered, as the div does not need to be recreated.
That’s better, like Saq said. But, be careful: Inside a loop, you only want to create things that vary based on the loop. IOW, don’t re-create things that are invariant during the loop:
Very contrived example but I think you get the issue: you don’t need to recreate x for each loop iteration (perhaps not even the div, depending on your requirements).
I echo what @CodaCoder illustrates in a loop by emphasising that to use div and other html elements you are effectively providing a display element, so you use them last. They alter the final output because they are not part of the structure or logic.