All great design, in any medium, involves learning how to work with the grain instead of against it.
These mock designs almost always focused on pixel perfectness, which meant trying to bend and twist the web to make it so. Spacer pixels, remember those? We were trying to make the raw materials of the web, particularly HTML, then latter CSS, do things they didn’t want to do. Things they weren’t meant to do.
What’s great about the ubiquity of the web is that designers don’t necessarily have to write HTML/CSS anymore in order to work with the grain. Purely visual tools today like Sketch and Figma already mirror the way the web works better than ever before.
For example, these tools are now able to model responsive designs and allow the user to decide how and when elements should respond when they are resized. This isn’t the web — but it’s closer to the web and keeps getting closer. Designers today who don’t know HTML/CSS can still work with the grain if they do their due diligence.