Ask HN: How Should Designers Make Mockups?

Hello, at my current company we have a really great designer that sadly doesn't have a formal training in web development. This makes it really hard for him to develop usable and responsive interfaces for our products. What I'm wondering is: is there some conceptual framework or a straight up product that would allow him to pay more attention on how elements behave with different widths, different content lengths and such? Something that would avoid the more basic errors of translation from SVG to HTML/CSS? Thanks