The key observation is a) responsive web pages often use large images and rely on the browser to scale them down; b) browsers do a bad job of resizing compared

The key observation is a) responsive web pages often use large images and rely on the browser to scale them down; b) browsers do a bad job of resizing compared to e.g. Photoshop. This is true but I don't think it's a very big problem. Most recent phones have ridiculous high screen resolutions so the blurring is completely invisible. Desktop PCs tend to have large screens, so not much downscaling is needed, and high-DPI screens are getting more common there too. It's definitely a good idea to use SVGs for line art, if possible. And browsers ought to use better resizing filters, it's not exactly rocket science. But with a high-DPI screen, even low-quality resizing looks fine.