It based on font-size 14px but I don't know why does Bootstrap set line-height to this value? Thank in advance.
Because it's trying to set the line height to 20px. A non-metric line-height value is the ratio of height to font size. Try it, divide 20 by 14. In scaling your fonts + leading, Bootstrap is amazingly stupid; setting values impossible for humans to grok in fullness or to adjust sanely. Throw that silliness out, and learn to scale for yourself. It will require some thinking on your part, but it will improve the typography of your sites. cheers, gary
To go along with bloated, inefficient and marginal on standards. If you really want to do web development; do the world a favor and learn to write your own code instead of using something as obtuse as Bootcrap.
To go with the others, bootcrap is idiotic halfwit mouth-breathing farm fresh grade A prairie pies; do yourself a favor and find a stick to scrape it off with. You are being led down the garden path to failure by the hot, trendy and ultimately useless dumbass garbage framework with it's presentational use of classes, endless pointless DIV and classes for nothing, and general "semantics, what's that?" development. ... but then, declaring 14px instead of using an EM measurement is equally silly; nothing like pissing all over accessibility. Though to be fair I say that 99.9999% of HTML, CSS and JS frameworks are idiotic halfwit mouth-breathing farm fresh grade A manure, wondering how anyone with more than two brain cells to rub together could possibly consider ANY of them to serve a legitimate purpose!