Working in CSS often means using EM for components. It is possible to switch between both PX and EM, and to change the base font value.
Linear isn't a browser extension, so it doesn't add a CSS overlay to your page. It's super easy to have rulers open while tweaking values in the inspector.
Get the proportions right once, then copy the rulers to make sure your margins are correct.
Toggle rulers using a global shortcut.
Working on a white background? Juste change the theme of the ruler.
Create as many rulers to make sure all your elements are aligned correctly.