This is an odd thing to say about the backend choices. Spring is ... well let's just say there are a few folks in and around that area.
This is precisely where the publisher has the most control over the user experience. Putting load on the browser makes a user's experience dependent on their hardware & software stack.
If the key to a good user experience was server side control, than the hallmark of a good website would be being an RDP stream and prefers-color-scheme wouldn't be a CSS feature but a HTTP header.
Yes, these are features implemented by every popular browser via HTML & CSS. Fancy front-end work frequently breaks these features.
> key to a good user experience was server side control
I think you're reading my comment from an extremely front-end perspective. I simply mean that where possible it's better to render pages and do logic on the server side versus on the client side. The same HTML + CSS is generated either way, the only question is what % of the work is done by the user's device vs what % is done in a data center.