Customised CSS rendering differently in editor than on live website

Site URL: https://www.ossability.com/

Hi!

I have been working on a website and customising it with CSS and HTML. It has gone live, but some of the features, specifically the blue / yellow underlines on the headings,  are placed perfectly in the editor seem to have shifted and moved on the live website. I'm not sure why as it still displays fine in the editor and in the preview.  

NOTE: We have since temporarily changed the code to look good on the live website, but in turn it messes up the formatting in the editor (as seen below).
This is seen in the editor and the preview
This is seen on the live website (Desired)
This is the code used to make the blue underlines:

.underline-blue::before{
  	background:#28aae1;
    content: "";
    position: absolute;
    bottom: 42px;
    height: 5px;
    width: 70px;
}

Does anyone have any advice on this? Why has this happened, and how can I make my editor the same as the live website?

Also, the lines move around a little depending on the size of the browser window (on laptop), and size of the screen (iPhones, tablets etc). Does anyone know why this happens, how, and can it be fixed?

Thank you!

Have you set position: relative for .underline-blue yet?

.underline-blue{position: relative; line-height:1;}