Below is an example rmarkdown html page that fixes the title while scrolling. Since the fixed header has a set height of 75px, the "Start content here" line has a top margin set to 75px so it doesn't get covered.
As for references, I've found https://www.w3schools.com/css/ to come in handy.
Note: the code chunk in the rmarkdown below was shortened to two tick marks `` in order to render in one block here. If you copy and paste this into an rmarkdown document, be sure to add the third tick marks.
---
title: "My Title"
output: html_document
---
``{css formatting, echo=FALSE}
#header {
position: fixed;
width: 100%;
height: 75px;
background-color: white;
}
``
<h3 style="margin-top: 75px;"> Start content here</h3>
`r rep('Content...<br>', 100)`
