Multi-step-form using html, css and js

Changing layout between desktop and mobile devices was hard for me. It could have been easier if I could change HTML structure depends on the device. For example, use flex box consists of two divs, one has sidebar, others have input area and buttons. This way I can easily layout these elements. However, on mobile devices buttons shouldn't be included in a div with input area to achieve the provided layout. So I was wondering if it's possible and okay to change HTML structure according to devices (screen size).