move a step, add some rationale

Author: annevk

fetch.bs

@@ -6052,7 +6052,7 @@ if the script checks that the URL has the right hostname.
 <h2 id=data-urls><code>data:</code> URLs</h2>
 
 <p>For an informative description of <code>data:</code> URLs, see RFC 2397. This section replaces
-that RFC's normative processing requirements. [[RFC2397]]
+that RFC's normative processing requirements to be compatible with deployed content. [[RFC2397]]
 
 <p>A <dfn><code>data:</code> URL struct</dfn> is a <a>struct</a> that consists of a
 <dfn for="data: URL struct">MIME type</dfn> (a <a for=/>MIME type</a>) and a
@@ -6074,14 +6074,14 @@ that RFC's normative processing requirements. [[RFC2397]]
  <li><p>Let <var>mimeType</var> be the result of <a>collecting a sequence of code points</a> that
  are not equal to U+002C (,), given <var>position</var>.
 
+ <li><p><a>Strip leading and trailing ASCII whitespace</a> from <var>mimeType</var>.
+
  <li><p>If <var>position</var> is past the end of <var>input</var>, then return failure.
 
  <li><p>Advance <var>position</var> by 1.
 
  <li><p>Let <var>encodedBody</var> be the remainder of <var>input</var>.
 
- <li><p><a>Strip leading and trailing ASCII whitespace</a> from <var>mimeType</var>.
-
  <li><p>Let <var>body</var> be the <a>string percent decoding</a> of <var>encodedBody</var>.
 
  <li>