I faced the same problem on Safari on Mac 10.14.6
What I ended up doing is using the “reader” feature of safari. You can find it on the left side of the address field of the page, when you click it you get a text based version of the website. All images and videos still appear in body, but the website formatting is stripped out.
Once in reader mode you can click on the right hand side “AA” icon to change the font style, size and color scheme to suit your preferences
This way I could go through the whole lesson, only downside is you need to get out of reader vision and back to top in order to find and click the “next” button.