DOM not API dependency updates

Author: VesaJuvonen

docs/spfx/extensions/get-started/build-a-hello-world-extension.md

@@ -15,6 +15,9 @@ You can also follow the steps in this article by watching the video on the Micro
 
 > [!Video https://www.youtube.com/embed/DnfRIl2YN8g]
 
+> [!IMPORTANT]
+> The SharePoint page [HTML DOM](https://www.w3.org/TR/WD-DOM/introduction.html) is not an API. You should avoid taking any dependencies on the page DOM structure or CSS styles, which are subject to change and potentially break your solutions. SharePoint Framework provides a rich API to customize the SharePoint experience in reliable ways and is the only supported means to interact with the SharePoint page HTML DOM.
+
 ## Create an extension project
 
 1. Create a new project directory in your favorite ___location.

docs/spfx/extensions/overview-extensions.md

@@ -18,6 +18,9 @@ SPFx Extensions enable you to extend the SharePoint user experience within moder
 
 You can build extensions alongside common scripting frameworks, such as Angular and React, or to plain JavaScript projects. For example, you can use React along with components from Office UI Fabric React to create experiences based on the same components used in Microsoft 365.
 
+> [!IMPORTANT]
+> The SharePoint page [HTML DOM](https://www.w3.org/TR/WD-DOM/introduction.html) is not an API. You should avoid taking any dependencies on the page DOM structure or CSS styles, which are subject to change and potentially break your solutions. SharePoint Framework provides a rich API to customize the SharePoint experience in reliable ways and is the only supported means to interact with the SharePoint page HTML DOM.
+
 ## See also
 
 - [Overview of the SharePoint Framework](../sharepoint-framework-overview.md)

docs/spfx/sharepoint-framework-overview.md

@@ -34,6 +34,9 @@ The runtime model improves on the Script Editor web part. It includes a robust c
 
 If you work primarily with C#, you want to learn more about client-side JavaScript development. Most of your existing JavaScript knowledge related to SharePoint, however, is transferable, as the data models haven't changed, and you’ll use the same [REST services](../sp-add-ins/get-to-know-the-sharepoint-rest-service.md) depending on your requirements. If you're a C# developer, TypeScript is a nice transition into the JavaScript world. The choice of IDE is up to you. Many developers like to use the cross-platform IDE [Visual Studio Code](https://code.visualstudio.com).
 
+> [!IMPORTANT]
+> The SharePoint page [HTML DOM](https://www.w3.org/TR/WD-DOM/introduction.html) is not an API. You should avoid taking any dependencies on the page DOM structure or CSS styles, which are subject to change and potentially break your solutions. SharePoint Framework provides a rich API to customize the SharePoint experience in reliable ways and is the only supported means to interact with the SharePoint page HTML DOM.
+
 > [!NOTE]
 > All supported scenarios for SharePoint Framework are included in the documentation. Any scenario which is not covered in the documentation, is considered as unsupported. If you need to validate the supportability of your planned scenario, please use the available Microsoft support channels.