I can recount my early days of programming, a world where HTML, CSS and JS weren't invented yet. All apps were built using Windows APIs and/or Microsoft Foundation Classes. Did we have such debates then?
Well, no. Designers did their design in Photoshop or equivalent, and it was the job of the developer to translate it into code. Whether they used resource files (equivalent of HTML/CSS) or created widgets on the fly (equivalent of runtime creation of DOM elements using javascript) was not of concern to the designer. Some designers even created resource files themselves, but they did not need to know how to code, the resource editor did that for them. This saved both the designer and the developer some time and prevented transmission errors.
Fast forward to today. One problem is that HTML/CSS/JS was invented for laying out WWW content and hyperlinking, not for apps with great UI. Tools (equivalent of the resource editor) don't exist, and laying out UI has become more complicated, requiring code.
I believe HTML and CSS is code, and therefore, not the forte of designers. It's the job of developers to convert design to CSS or React Components or whatever. Only some developers will be good at this. That's reality. Others will be called backend engineers.
The same was applicable 30 years back. If designers messed with resource files, they ran the risk of breaking code that assumed something, like resource IDs and a particular hierarchy of components. So, designers who did generate resource files didn't touch it after they handed over the initial version.
If your designers produce markup and CSS, great. That's good for all. If they want to mess with it after the developers have integrated it, I'd say they are brave. In which case, they should be brave enough to learn to do it the React way if required.
But I'd really avoid it, and hire front-end developers instead.