Pixels are static values which may work for certain dimensions but will cause problems and hinder design at different dimensions/views(mobile-desktop)
For responsive web design it is a must to follow certain design practices :
1- No Pinch Zoom: Solved by applying viewport correctly.
2- No Horizontal Scrolling: Solved by using relative sizing and positioning and instead of absolute positioning.
3- Font-size Handling: Use em and rems instead of pixels.
4- Layout: Use Media Queries,Flexboxes, Bootstrap etc.