I’m a firm believer that if you’re designing for the web you should have at least some idea of how to build for the web. For my money, nothing beats spending time with developers to cultivate your understanding of how things are put together. Indeed, such conversations should directly affect how you structure your design files. Sketch is great for this. With its overridable symbols you can actively design in a way that lends itself to translation into HTML and CSS.
It seems to me that this is often a missing piece in a designer’s toolbox. All too often, a designer will fully understand about using a grid system, or creating small reusable components. But the approach in creating these isn’t based on thinking like a developer. Even when it is, assumptions are made during design time but never actually discussed with the development team until “handover”.
If you’re designing anything for the web, and you’re not building it yourself: Collaborate early with those that are.