Put the names that the validator uses for the boilerplate into the sp…

…ec (ampproject#3273)

* Put the names that the validator uses for the boilerplate into the
link and headline for the AMP Boilerplate. This should help people
if they're looking for it with CTRL+F or search engines.

* add clarifying comment

* tweaks

* use dom selectors style

* amp-boilerplate

spec/amp-boilerplate.md

@@ -14,7 +14,15 @@ See the License for the specific language governing permissions and
 limitations under the License.
 -->
 
-# AMP Boilerplate Code
+# AMP Boilerplate Code ('head > style[amp-boilerplate]' and 'noscript > style[amp-boilerplate]')
+
+AMP HTML documents must contain the following boilerplate in their `head` tag.
+Validation is currently done with regular expressions, so it's important to keep
+mutations as minimal as possible. Currently, allowed mutations are: (1)
+inserting arbitrary whitespace immediately after the `style` tag opens, and
+immediately before it closes; (2) replacing any space in the snippet below with
+arbitrary whitespace.
+
 ```
 <style amp-boilerplate>body{-webkit-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 8s steps(1,end) 0s 1 normal both;animation:-amp-start 8s steps(1,end) 0s 1 normal both}@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}</style><noscript><style amp-boilerplate>body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}</style></noscript>
 ```

spec/amp-html-format.md

@@ -98,7 +98,7 @@ AMP HTML documents MUST
 - <a name="chrs"></a>contain a `<meta charset="utf-8">` tag as the first child of their head tag. [🔗](#chrs)
 - <a name="vprt"></a>contain a `<meta name="viewport" content="width=device-width,minimum-scale=1">` tag inside their head tag. It's also recommend to include `initial-scale=1` (1). [🔗](#vprt)
 - <a name="scrpt"></a>contain a `<script async src="https://cdn.ampproject.org/v0.js"></script>` tag inside their head tag. [🔗](#scrpt)
-- <a name="boilerplate"></a>contain the [AMP boilerplate code](amp-boilerplate.md) in their head tag. [🔗](#boilerplate)
+- <a name="boilerplate"></a>contain the [AMP boilerplate code ('head > style[amp-boilerplate]' and 'noscript > style[amp-boilerplate]')](amp-boilerplate.md) in their head tag. [🔗](#boilerplate)
 
 (1) `width=device-width,minimum-scale=1` is required to ensure [GPU rasterization](https://www.chromium.org/developers/design-documents/chromium-graphics/how-to-get-gpu-rasterization) is enabled.