That is because Bootstrap team (Twitter team) has invested years of experience and developer-power in the development of this framework to make it,
* Developer-friendly
* Cross-browser
* Responsive
* Stable
You cannot take every browser into account, and then provide a design that provides all these features and — even if that does happen — does not takes months of foundation development, including JavaScript and CSS core.
Thus, it is recommended to
not build it by yourself using HTML, CSS and JavaScript, instead just go with usage of jQuery and Bootstrap.
On a side note, if this is a learning project, please consider my advice, provided above. But if you still want to try it out, following are the ways to go,
Using media queries - CSS: Cascading Style Sheets | MDN[
^]
HTML forms - Learn web development | MDN[
^]
These are the two documentations about the W3 based standards to control the responsiveness of your app, as well as to control how users interact and input values to the forms, and also how forms take the input into account — validation, processing, masking etc.