Add extractor and builder

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snippets/focus-within.md

@@ -5,8 +5,6 @@ tags: visual, interactivity,intermediate
 
 Changes the appearance of a form if any of its children are focused.
 
-#### HTML
-
 ```html
 <div class="focus-within">
   <form>
@@ -16,8 +14,6 @@ Changes the appearance of a form if any of its children are focused.
 </div>
 ```
 
-#### CSS
-
 ```css
 form {
   border: 3px solid #2d98da;
@@ -31,8 +27,6 @@ form:focus-within {
 }
 ```
 
-#### Demo
-
 <!-- Leave this blank, the build script will generate the demo for you. -->
 
 #### Explanation