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<html lang="ja"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>tryjquery_css_width_ref ソース</title> <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"> </script> <script> $(document).ready(function(){ $("button").click(function(){ alert("Width of div: " + $("div").width()); }); }); </script> </head> <body> <div style="height:100px;width:300px;padding:10px;margin:3px;border:1px solid blue;background-color:lightblue;"></div><br> <button>div の幅を表示</button> </body> </html>