Add note about Cumulative Layout Shift

Author: mdingena

docs/server-side-rendering.md

@@ -1,8 +1,16 @@
 # Server-Side Rendering
 
-The values returned from [`useBreakpoints()`](api.md#usebreakpoints) default to `undefined`, which is the case when:
+The `widthMatch` and `heightMatch` values returned from [`<Observe>`](api.md#observe) and [`useBreakpoints()`](api.md#usebreakpoints) default to `undefined`. This is the case when:
 
-* the observed min-size isn't specified in your [`options`](api.md#widths);
-* rendering a component server-side.
+- the observed min-size isn't specified in your [`options`](api.md#usebreakpoints);
+- rendering a component server-side.
 
-You can use this `undefined` value to display your component differently for SSR purposes. How you do it is up to you (loading component, default CSS styles, placeholder content, `null`, etc).
+You can use this `undefined` value to display your component differently for SSR purposes. How you do it is up to you (loading spinner component, default CSS styles, placeholder content, `null`, etc).
+
+# ⚠️ Beware of [Cumulative Layout Shift](https://web.dev/cls/)
+
+Remember, this is a JavaScript solution to a CSS problem. If you use React Breakpoints to apply different styles to your component based on their size, and you are rendering some HTML server-side, you may end up applying styles to your components that do not match their computed styles once JavaScript loads on the client. This means you could introduce a flash of incorrectly styled content (FOISC?).
+
+Unfortunately, there is no easy way around it: this is the nature of the beast. If you care deeply about [CLS](https://web.dev/cls/) (and for public pages you probably should!), you need to keep this side-effect in mind.
+
+However, React Breakpoints truly shines when you're building highly-responsive dashboards with graphs and tables, each individually responsive and aware of their own sizes. Dashboards for signed-in users generally don't suffer from SEO penalties like CLS, because they are not indexed by search engines.