The reason why these prepositions here are used and why "be" form is used in this youtube information passage

Question

sentence A: Many YouTube videos will play using HTML5 in supported browsers.

sentence B: You can request that the HTML5 player be used if your browser doesn't use it by default.

Hello. The above passage is from a youtube page. I made the words bold which I am about to ask about.

I have 3 question about that passage.

Question 1) In the sentence A, doesn't the part "using~" mean a method or a way of playing? Then, why is "by using" not used instead of "using" there?

Question 2) In the sentence B, why is "be" used instead of just "is"? Is there any word omitted before "be", for example, "should" or "may"? I wish to get an explanation about this.

Question 3) In the sentence B, why is "by" used before "default"? I think "as" or "for" are much more appropriate to it. Or all of them are usuable here?

P.S. I always thank you so much for your sincere, brilliant helps. Even a very short, simple explanation is very helpful to me.

Answer 1

1. Either using or by using would work here. In this case, the whole phrase "using HTML5 in supported browsers" acts as an adverb modifying play. Adverbs answer the question, "How?" In other words, this sentence is saying

"How will the video play?"

"Using HTML5 in supported browsers."

2. This is an example of the (somewhat rare) subjunctive mood in English. It is often used when expressing a desire, hope, hypothetical scenario, etc.

E.g. (subjunctive in bold):

"I wish that I were taller."

"He insisted that I be quiet."

3. "By default" is an idiomatic phrase. "For default" or "as default" are arguably also okay, but they sound weird because they are not idiomatic.