There are some promising moves that direction (such as responsive resizing in Sketch and projects to automatically convert sketch files to React components), but we're not there yet—most designers I know are still producing largely static designs that have to be put through development cycles.
And I know you can live design with HTML and CSS, skipping the static mockup phase. That's my preferred method at the moment, but I'm here to tell you it's still pretty slow and tedious and makes me long for better web design tooling.