docs: add a warning for Customizable select elements (#40400)

* docs: add a warning for `Customizable select elements`

* Update files/en-us/learn_web_development/extensions/forms/customizable_select/index.md

* Update files/en-us/learn_web_development/extensions/forms/customizable_select/index.md

---------

Co-authored-by: Chris Mills <[email protected]>

Author: paoloricciuti

files/en-us/learn_web_development/extensions/forms/customizable_select/index.md

@@ -8,7 +8,10 @@ sidebar: learnsidebar
 
 {{PreviousMenuNext("Learn_web_development/Extensions/Forms/Advanced_form_styling", "Learn_web_development/Extensions/Forms/UI_pseudo-classes", "Learn_web_development/Extensions/Forms")}}
 
-This article explains how to use dedicated, modern HTML and CSS features together to create fully-customized {{htmlelement("select")}} elements. This includes having full control over styling the select button, drop-down picker, arrow icon, current selection checkmark, and each individual {{htmlelement("option")}} element.
+This article explains how to create fully-customized {{htmlelement("select")}} elements using experimental browser features. This includes having full control over styling the select button, drop-down picker, arrow icon, current selection checkmark, and each individual {{htmlelement("option")}} element.
+
+> [!WARNING]
+> The CSS and HTML features demonstrated in this article currently have limited browser support; check the browser compatibility tables on the individual feature reference pages for more details. Some JavaScript frameworks block these features; in others, they cause hydration failures when Server-Side Rendering (SSR) is enabled.
 
 ## Background