Responsive component using html, css, js

This looks very good! Only a little off from the design.

The answers to your questions are pretty subjective but here is what I do:

For mobile view I usually play around with the responsive design on google developer tools and set media queries where things start to break. I also us Sizzy to see several devices at once while I'm developing.

The reset css I usually use is

:root {
    font-size: 10px;
}

*, *::before, *::after {
    margin: 0;
	padding: 0;
	border: 0;
	font-size: 100%;
	font: inherit;
	vertical-align: baseline;
    z-index: 0;
    box-sizing: border-box;
} ```