W3C Internationalization Checker

このチェッカは、国際化への配慮のレベルを決定するためにWebページ上で様々なテストを実行します。また、詳細については、概要ページを参照してくださいこのような文字エンコーディングと言語の宣言、などのページに関する主要な国際情報を、まとめたものです。

• W3C I18n Checker

Explanation

This page currently uses the following XML declaration:

<?xml version="1.0" encoding="utf-8"?>

XML declarations are sometimes used with XHTML pages that are served as text/html, so that when those files are read by an XML parser, rather than an HTML parser, the encoding information is recognized. However, an XML declaration in an HTML document can cause Internet Explorer to render in quirks mode rather than standards mode, so it is generally recommended that you avoid its use for such hybrid documents. If you use UTF-8 you don't need an XML declaration for an conforming XML parser.
What to do

Since you are using XHTML 1.x but serving it as text/html, use UTF-8 for your page and remove the XML declaration.

UTF-8ならxml宣言をする必要はないらしい