jQueryのロードのフォールバックでHTML4やXHTML1.0準拠の書き方を考えてみた

InitializrのHTMLとXHTML1.0の文法エラー

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/libs/jquery-1.7.2.min.js"><\/script>')</script>

CSS TricksのjQueryのフォールバックの書き方

<script type="text/javascript" src="http://ajax.microsoft.com/ajax/jquery/jquery-1.4.2.min.js"></script>
<script type="text/javascript">
if (typeof jQuery == 'undefined') {
    document.write(unescape("%3Cscript src='/js/jquery-1.4.2.min.js' type='text/javascript'%3E%3C/script%3E"));
}
</script>

Google Analyticsのコードを参考にしてみる

<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
</script>
try{
var pageTracker = _gat._getTracker("UA-xxxxxx-x");
pageTracker._trackPageview();
} catch(err) {}
</script>

<script type="text/javascript">

  var _gaq = _gaq || [];
  _gaq.push(['_setAccount', 'UA-XXXXX-X']);
  _gaq.push(['_trackPageview']);

  (function() {
    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
  })();

</script>

結論：最終コード（Google Analytics的コードの場合）

<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script type="text/javascript">
    (function() {
        window.jQuery ||
            var jq = document.createElement('script');
            jq.type = 'text/javascript';
            jq.async = true;
            jq.src = '/js/jquery-1.7.2.min.js';
            var s = document.getElementsByTagName('script')[0];
            s.parentNode.insertBefore(jq, s);
    })();
</script>