I love what Bret Victor says about this in his Magic Ink paper [1]. Replace "information software" with "web applications" as you read it:
"I suggest that the design of information software should be approached initially and primarily as a graphic design project. The foremost concern should be appearance—what and how information is presented. The designer should ask: What is relevant information? What questions will the viewer ask? What situations will she want to compare? What decision is she trying to make? How can the data be presented most effectively? How can the visual vocabulary and techniques of graphic design be employed to direct the user’s eyes to the solution? The designer must start by considering what the software looks like, because the user is using it to learn, and she learns by looking at it."
The act of creating a solution IS the hard part, not tweaking CSS properties or knowing how to integrate Bootstrap components just right. All those things should just get out of the way and let creatives do the important work, without bogging down in the implementation details.
In case you're interested, I wrote a bit more about this on our blog a while ago: http://blog.webflow.com/designers-will-rule-the-web