I know HTML but not know boorstrap is it works......

Question

html.css

Answer 1

Download Bootstrap and read the MD (readme files) and online tutorials. There is also code playground on google play.
Bootstrap is the best. If you dont use it you may be a loser.

Answer 2

https://www.sololearn.com/learn/1051/?ref=app
Finish the HTML and CSS courses. There's a tiny bit of JavaScript in Bootstrap, but otherwise, it's a glorified premade stylesheet (in my opinion). There are others out there. Companies who want to minimize their ramp up time on the stylesheets use Bootstrap (or something similar, at their discretion). Remember also that Bootstrap is from Twitter and is mobile first, so if your use case would be better off with a desktop first setup, you may find it extra work to use a tool where the priorities are reversed.