Introduction to HTML and CSS course- Styles.css missing lines of code & serious performance issues on Workspaces

Hi everyone,

I've just started this course, but I'm having two issues- firstly the performance on my PC is incredibly poor. I'm running on an i7 CPU with no prior issues on Java courses, but here workspaces is lagging, leading Firefox to use 30%+ of my CPU, and ever increasing amounts of ram (topping out at over 96% of the 16GB available?!).

Secondly, despite reloading a fresh copy of the workspace several times, it only contains the CSS file up to line 77. Everything else after is blank!

Is this a known issue with the course?

EDIT: I've sorted the missing CSS lines by copying those missing from the file included in the downloads file. Currently, workspaces doesn't seem to be going too crazy, so perhaps the missing lines of CSS were causing workspaces to act up?

EDIT 2: okay so it seems to the lines aren't missing after all. When I copy them across from the file in the downloads tab, everything is obviously there, I can scroll up and down the lines of code with no issue. However, when I then go to play the video tutorial and go back to scrolling up and down the CSS code in workspaces, everything that wasn't on my screen (above or below) when I pressed play appears as blank white space with the usual workspace panels remaining visible. Workspaces then becomes increasingly laggy, and so does the window in which I have the video tutorial open. Until I close down either of the two Firefox windows, both remain laggy and unresponsive (for instance I can't pause or select a timestamp on the video).

Edit 2: everything seems to be working fine in Microsoft Edge, so it might be a Firefox specific issue?