@rkaplan you could capture the PDF of a webpage (see various scripts I’ve posted here such as Automatically capture and annotate items (to use with Obsidian)) and convert the PDF into an image if you’d like. For what you want I would use PDF anyway because it has the added benefit of the text layer. But e.g. with your Outlook e-mails why not just import them as .eml to DT?
If you want a good screenshot tool that does what you want (+more like OCR, create GIFs etc) take a look at Cleanshot X