Personally, I really like the way you organized your HTML. It feels readable, well-organized, and evenly spaced. The overall structure and approach in your CSS seems great as well. I think the overall is good enough that the only complaint I have are very small details like the following:
The font weight for the publish date should probably be around 500, as suggested by the style guide.
The line height could be a bit higher.
The padding a round the main content could be a bit higher.
but I'm only nitpicking because I couldn't find anything else.
The above suggestions are just minute details compared to the well-structured nature of the code you wrote so I probably wouldn't worry about it. I think this is great!
I think the code looks great overall. However, the solution seems slightly different aesthetically. The text should take up less width than the QR code. There seems to be minor capitalization errors. There seems to be lots of reset codes but I think small projects like this may not need many resets. The HTML is spaced a bit awkwardly and might be more readable with better spacing? But it's just solely my opinion and could be wrong.