Being so used to various "draw a box around this portion of your screen to capture it" tools, I didn't even notice browsers have started to offer this functionality directly.
Navigate to the desired element in your browser's inspector in your preferred manner and then right-click it. One of the options will be to screenshot it ⸺ at least in Firefox and Chrome. I'm too lazy to check anything else, and you should be using Firefox for your personal browsing anyway.
Here's an example I took showing the beginning of our downfall:
Since your browser (and tools like Photoshop) don't output optimized PNG files, you'll want to run them through some kind of optimization system afterward. There are more than plenty of them online, or you could download an app and drag-drop the images onto it.