Which unit should I use when editing css in the mobile phone h5 page?

recently, UI gave a picture of H5. The unit above is px, but it will be messy to make it on mobile phones like this. After all, a px on some mobile phones is not really 1px.
the question is: UI thinks we should give px the corresponding size. How can I convince UI, to let her believe me and change it to rem, or some other better way

?.
Mar.04,2021

find her two cell phones of different sizes. According to her design, write the width of the page and show her the white margin.


computer browser opens the mobile phone Tmall to show him the css unit inside, that is, the em used


Thank you for your advice

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