1. No, MUI’s sx prop does render styles as inline CSS. It renders a random unique class name, which will change if you add or remove some property in the sx object. Actually, it is better than writing inline-css, because class names are better for SEO and performance. Check the image below where I write styling in three different ways, and check its output:

2. No, it won’t affect SEO as long as you use MUI AppCacheProvider with NextJs. Check this documentation to implement AppCacheProvider.

3. There is no bad impact if you config your NexJs app as per the MUI’s NextJs documentation.

4. These libraries also generate class-based styles or scoped CSS and are excellent for SEO and performance. I think If you’re already using MUI and the sx prop, switching libraries isn’t necessary.

💡I hope it helps!

1. For First Question , Not realy!
   with sx props , you can work with The theme , it’s look like MUI System with sx props ,change the theme for that specific component
   2- For second Question , MUI Using Utility Classes behind the scenes , you can track them with dev tools , MUI manage almost every thing with Utility Classes , But you must consider that , MUI using React and The React it self is not that SEO friendly!

2. For Better SEO Compatibility , I recommend to use some framework like Tailwind because this framework it can be only dev-dependency , and the final result is only CSS , of course for more animation you need some other frameworks like motion , but however in animation MUI is not that good

3. But if for something like first initializing the page , use SSR , SEO of your website will be fine