Thanks for preparing a sample page to demonstrate the issue - it is really helpful for us.
To be honest, I am not familiar in depth with web design and some of the following assumptions may not be quite accurate. However, I managed to sort out that the REMs are a way of setting font-sizes based on the font-size of the root HTML element. I am not sure how these are affected by the css styles applied on a page, but I suspect that the different browsers are using and rendering all these in different way.
Here is what I found out with the help of the sample page - adding a verification step, to check the font size of the paragraph in question, is different in Internet Explorer and the rest of the browsers (Edge Beta and Firefox tested) - please see the attached screenshot. It seems that Test Studio detects differences in the DOM tree loaded in the different browsers and thus the step recorded in Firefox will not work against IE, and vice versa - the step recorded in IE will not work against the other browsers.
Still there is an option to workaround this inconsistent behavior - each step in Test Studio can be excluded from the test execution based on which is the browser to run the test against. This is a step property named RunsAgainst in which you can choose the specific browser to execute the step against. So, you can have the respective step for each browser you need to run the tests against.
Please, let me know if this information is helpful for you and if there is anything else you need to discuss.
Thank you for cooperation in advance.