[css-forms-1] Address errors from validator.w3.org

Author: nt1m

css-forms-1/Overview.bs

@@ -91,7 +91,8 @@ Markup Shorthands: css yes, markdown yes
 
   Here are the [=basic appearance=] colors inheriting respectively the default light and dark mode colors from the root element:
 
-  <img src="images/base-screenshot.png" style="width: 50%; height: auto;"><img src="images/base-screenshot-dark.png" style="width: 50%; height: auto;">  
+  <img src="images/base-screenshot.png" style="width: 50%; height: auto;" alt="Screenshot of the basic appearance with a light color scheme">
+  <img src="images/base-screenshot-dark.png" style="width: 50%; height: auto;" alt="Screenshot of the basic appearance with a dark color scheme">  
 
 ### Color/Font Customization
 
@@ -110,7 +111,7 @@ Markup Shorthands: css yes, markdown yes
     }
     </pre>
 
-    <img src="images/base-example-1.png" style="width: 100%; height: auto;">
+    <img src="images/base-example-1.png" style="width: 100%; height: auto;" alt="Screenshot of a customized basic appearance with brown text and a pale yellow background">
   </div>
 
   <div class="example">
@@ -127,7 +128,7 @@ Markup Shorthands: css yes, markdown yes
     }
     </pre>
 
-    <img src="images/base-example-2.png" style="width: 100%; height: auto;">
+    <img src="images/base-example-2.png" style="width: 100%; height: auto;" alt="Screenshot of a customized basic appearance with green text and a black background">
   </div>
 
 # Styling Pickers
@@ -967,7 +968,7 @@ At the limit, things like internal padding, border-radius, etc might be used.
 </blockquote>
 
 <div class='example'>
-  <img src=images/time01.png>
+  <img src=images/time01.png alt="iOS time picker">
 
   <blockquote cite="https://lists.w3.org/Archives/Public/www-style/2015Jan/0622.html">
     The black area is the button color; a very light transparency of it
@@ -976,7 +977,7 @@ At the limit, things like internal padding, border-radius, etc might be used.
 </div>
 
 <div class='example'>
-  <img src=images/time02.png>
+  <img src=images/time02.png alt="Android time picker">
 
   <blockquote cite="https://lists.w3.org/Archives/Public/www-style/2015Jan/0622.html">
     It's hard to tell without more context, but for the one on the right,