デフォルトで整形されてなかったので整形後を載せてみる．

http://gdata.youtube.com/feeds/api/videos?vq=紅蓮の弓矢&orderby=relevance&start-index=1&max-results=1

<?xml version='1.0' encoding='utf-8'?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:media="http://search.yahoo.com/mrss/" xmlns:opensearch="http://a9.com/-/spec/opensearchrss/1.0/" xmlns:gd="http://schemas.google.com/g/2005" xmlns:yt="http://gdata.youtube.com/schemas/2007">
    <id>
        http://gdata.youtube.com/feeds/api/videos
    </id>
    <updated>
        2014-01-06T12:07:20.460Z
    </updated>
    <category scheme="http://schemas.google.com/g/2005#kind" term="http://gdata.youtube.com/schemas/2007#video">
        <title type="text">
            Videos matching: 紅蓮の弓矢 linked horizon
        </title>
        <logo>
            http://www.gstatic.com/youtube/img/logo.png
        </logo>
        <link rel="alternate" type="text/html" href="http://www.youtube.com">
        </link>
        <link rel="http://schemas.google.com/g/2005#feed" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos">
        </link>
        <link rel="http://schemas.google.com/g/2005#batch" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos/batch">
        </link>
        <link rel="self" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos?start-index=1&amp;max-results=1&amp;vq=%E7%B4%85%E8%93%AE%E3%81%AE%E5%BC%93%E7%9F%A2+linked+horizon&amp;orderby=relevance">
        </link>
        <link rel="next" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos?start-index=2&amp;max-results=1&amp;vq=%E7%B4%85%E8%93%AE%E3%81%AE%E5%BC%93%E7%9F%A2+linked+horizon&amp;orderby=relevance">
            <author>
                <name>
                    YouTube
                </name>
                <uri>
                    http://www.youtube.com/
                </uri>
            </author>
            <generator version="2.1" uri="http://gdata.youtube.com">
                YouTube data API
            </generator>
            <opensearch:totalresults>
                14962
            </opensearch:totalresults>
            <opensearch:startindex>
                1
            </opensearch:startindex>
            <opensearch:itemsperpage>
                1
            </opensearch:itemsperpage>
            <entry>
                <id>
                    http://gdata.youtube.com/feeds/api/videos/1GzpHj4YxaE
                </id>
                <published>
                    2013-04-21T05:22:10.000Z
                </published>
                <updated>
                    2014-01-02T02:30:58.000Z
                </updated>
            </entry>
        </link>
    </category>
    <category scheme="http://schemas.google.com/g/2005#kind" term="http://gdata.youtube.com/schemas/2007#video">
    </category>
    <category scheme="http://gdata.youtube.com/schemas/2007/categories.cat" term="Film" label="映画とアニメ">
        <title type="text">
            Linked Horizon~「紅蓮の弓矢」~
        </title>
        <content type="text">
            進撃の巨人 OP.
        </content>
        <link rel="alternate" type="text/html" href="http://www.youtube.com/watch?v=1GzpHj4YxaE&amp;feature=youtube_gdata">
        </link>
        <link rel="http://gdata.youtube.com/schemas/2007#video.related" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos/1GzpHj4YxaE/related">
        </link>
        <link rel="http://gdata.youtube.com/schemas/2007#mobile" type="text/html" href="http://m.youtube.com/details?v=1GzpHj4YxaE">
        </link>
        <link rel="self" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos/1GzpHj4YxaE">
            <author>
                <name>
                    Natsuhi19HyNh
                </name>
                <uri>
                    http://gdata.youtube.com/feeds/api/users/Natsuhi19HyNh
                </uri>
            </author>
            <gd:comments>
                <gd:feedlink rel="http://gdata.youtube.com/schemas/2007#comments" href="http://gdata.youtube.com/feeds/api/videos/1GzpHj4YxaE/comments" counthint="85">
                </gd:feedlink>
            </gd:comments>
            <yt:hd>
                <media:group>
                    <media:category label="映画とアニメ" scheme="http://gdata.youtube.com/schemas/2007/categories.cat">
                        Film
                    </media:category>
                    <media:content url="http://www.youtube.com/v/1GzpHj4YxaE?version=3&amp;f=videos&amp;app=youtube_gdata" type="application/x-shockwave-flash" medium="video" isdefault="true" expression="full" duration="349" yt:format="5">
                    </media:content>
                    <media:content url="rtsp://r7---sn-o097zues.c.youtube.com/CiILENy73wIaGQmhxRg-Huls1BMYDSANFEgGUgZ2aWRlb3MM/0/0/0/video.3gp" type="video/3gpp" medium="video" expression="full" duration="349" yt:format="1">
                    </media:content>
                    <media:content url="rtsp://r7---sn-o097zues.c.youtube.com/CiILENy73wIaGQmhxRg-Huls1BMYESARFEgGUgZ2aWRlb3MM/0/0/0/video.3gp" type="video/3gpp" medium="video" expression="full" duration="349" yt:format="6">
                        <media:description type="plain">
                            進撃の巨人 OP.
                        </media:description>
                        <media:keywords>
                            <media:player url="http://www.youtube.com/watch?v=1GzpHj4YxaE&amp;feature=youtube_gdata_player">
                                <media:thumbnail url="http://i.ytimg.com/vi/1GzpHj4YxaE/0.jpg" height="360" width="480" time="00:02:54.500">
                                </media:thumbnail>
                                <media:thumbnail url="http://i.ytimg.com/vi/1GzpHj4YxaE/1.jpg" height="90" width="120" time="00:01:27.250">
                                </media:thumbnail>
                                <media:thumbnail url="http://i.ytimg.com/vi/1GzpHj4YxaE/2.jpg" height="90" width="120" time="00:02:54.500">
                                </media:thumbnail>
                                <media:thumbnail url="http://i.ytimg.com/vi/1GzpHj4YxaE/3.jpg" height="90" width="120" time="00:04:21.750">
                                    <media:title type="plain">
                                        Linked Horizon~「紅蓮の弓矢」~
                                    </media:title>
                                    <yt:duration seconds="349">
                                    </yt:duration>
                                </media:thumbnail>
                            </media:player>
                        </media:keywords>
                    </media:content>
                </media:group>
                <gd:rating average="4.4121213" max="5" min="1" numraters="660" rel="http://schemas.google.com/g/2005#overall">
                    <yt:statistics favoritecount="0" viewcount="511235">
                    </yt:statistics>
                </gd:rating>
            </yt:hd>
        </link>
    </category>
</feed>