Why I don't separate design and code
What changes when the person making the design is also building it.
I came into this from design. For a long time I was thinking about the layout, the branding, the type, and the details. Then I started getting more interested in what happened after the mockup.
Buttons have states. Content changes. Mobile layouts get weird. Animations that looked great in a comp don't always feel right in the browser. Code makes you deal with the actual experience.
Now I tend to think about both at once. I want it to look good, but I also want the behavior to make sense and the implementation to hold together.
I still go back and forth a lot. That's not a broken process. That's usually how the better ideas show up.