docs: clarify dev browser support version (vitejs#20127)

Author: sapphi-red

docs/guide/index.md

@@ -20,7 +20,7 @@ You can learn more about the rationale behind the project in the [Why Vite](./wh
 
 ## Browser Support
 
-During development, Vite sets [`esnext` as the transform target](https://esbuild.github.io/api/#target), because we assume a modern browser is used and it supports all of the latest JavaScript and CSS features. This prevents syntax lowering, letting Vite serve modules as close as possible to the original source code.
+During development, Vite assumes that a modern browser is used. This means the browser supports most of the latest JavaScript and CSS features. For that reason, Vite sets [`esnext` as the transform target](https://esbuild.github.io/api/#target). This prevents syntax lowering, letting Vite serve modules as close as possible to the original source code. Vite injects some runtime code to make the development server work. These code use features included in [Baseline](https://web-platform-dx.github.io/web-features/) Newly Available at the time of each major release (2025-05-01 for this major).
 
 For production builds, Vite by default targets [Baseline](https://web-platform-dx.github.io/web-features/) Widely Available browsers. These are browsers that were released at least 2.5 years ago. The target can be lowered via configuration. Additionally, legacy browsers can be supported via the official [@vitejs/plugin-legacy](https://github.com/vitejs/vite/tree/main/packages/plugin-legacy). See the [Building for Production](./build) section for more details.