Add appearance:base-select to select explainer

Opt-ins to the new select have been discussed here:
openui#985

site/src/pages/components/selectlist.mdx

@@ -40,9 +40,7 @@ The [2020 MDN browser compatibility report had feedback](https://mdn.dev/archive
 
 ## Opting in to the new behavior
 
-The `<select>` element will continue to behave as it currently does unless a `<button>` or `<datalist>` element is provided as a child of the `<select>`. `<button>` elements replace the native button which opens the listbox, and `<datalist>` replaces popup listbox. By replacing the native components with regular web content, the contents are fully stylable and customizable.
-
-The current behavior in the HTML parser for `<select>` is to remove all tags which aren't `<option>` or `<optgroup>`. We will change the HTML parser to allow `<datalist>` and `<button>` as a child of `<select>`, and allow any other content within that `<datalist>` or `<button>`.
+The `<select>` element will continue to behave as it currently does unless it has the `appearance:base-select` CSS property. Previously proposed opt-ins included a new tag name, an additional HTML attribute, and the presence of a child `<button>` or `<datalist>`.
 
 Here is a basic `<select>` element today:
 ```html
@@ -54,7 +52,19 @@ Here is a basic `<select>` element today:
 
 And here the same `<select>` with opt ins to the new stylability behavior which this explainer proposes:
 ```html
-<select>
+<select style="appearance:base-select">
+  <option>one</option>
+  <option>two</option>
+</select>
+```
+
+## HTML parser changes
+
+The current behavior in the HTML parser for `<select>` is to remove all tags which aren't `<option>` or `<optgroup>`. We will change the HTML parser to allow `<datalist>` and `<button>` as a child of `<select>`, and allow any other content within that `<datalist>` or `<button>`. The `<button>` allows authors to replace the in-page button which opens the popup, and the `<datalist>` allows authors to replace the popup element containing the options.
+
+Here is a basic example which uses the parser changes:
+```html
+<select style="appearance:base-select">
   <button>
     <selectedoption></selectedoption>
   </button>
@@ -534,3 +544,6 @@ The HTML parser will not allow `<button>` or `<datalist>` children when the `mul
 - [Why not make selection follow focus](https://github.com/openui/open-ui/issues/742)
 - [Why not allow select or datalist in size/multiple select](https://github.com/openui/open-ui/issues/977#issuecomment-1910874971)
 - [Why `<button type=popover>`](https://github.com/openui/open-ui/issues/939#issuecomment-1910837275)
+- Why opt-in with CSS
+  - https://github.com/whatwg/html/issues/9799#issuecomment-1789202391
+  - https://github.com/whatwg/html/issues/9799#issuecomment-1926411811