<b:widget id='Header1' locked='true' title='YOUR BLOG (Header)' type='Header' version='1' visible='true'>
<b:includable id='main'>
<b:if cond='data:useImage'>
<b:if cond='data:imagePlacement == "BEHIND"'>
<!--
Show image as background to text. You can't really calculate the width
reliably in JS because margins are not taken into account by any of
clientWidth, offsetWidth or scrollWidth, so we don't force a minimum
width if the user is using shrink to fit.
This results in a margin-width's worth of pixels being cropped. If the
user is not using shrink to fit then we expand the header.
-->
<b:if cond='data:mobile'>
<div id='header-inner'>
<div class='titlewrapper'>
<b:include name='title'/>
</div>
<b:include name='description'/>
</div>
<b:else/>
<a expr:href='data:blog.canonicalHomepageUrl'>
<amp-img expr:alt='data:title' expr:height='data:height' expr:id='data:widget.instanceId + "_headerimg"' expr:src='data:sourceUrl' expr:width='data:width'/>
</a>
<div class='titlewrapper'>
<b:include name='title'/>
</div>
<b:include name='description'/>
</b:if>
<b:else/>
<!--Show the image only-->
<div id='header-inner'>
<a expr:href='data:blog.canonicalHomepageUrl'>
<amp-img expr:alt='data:title' expr:height='data:height' expr:id='data:widget.instanceId + "_headerimg"' expr:src='data:sourceUrl' expr:width='data:width'/>
</a>
<!--Show the description-->
<b:if cond='data:imagePlacement == "BEFORE_DESCRIPTION"'>
<b:include name='description'/>
</b:if>
</div>
</b:if>
<b:else/>
<!--No header image -->
<div id='header-inner'>
<div class='titlewrapper'>
<b:include name='title'/>
</div>
<b:include name='description'/>
</div>
</b:if>
</b:includable>
<b:includable id='description'>
<div class='descriptionwrapper'>
<data:description/>
</div>
</b:includable>
<b:includable id='title'>
<b:if cond='data:blog.url == data:blog.homepageUrl'>
<h1 class='title'><data:title/></h1>
<b:else/>
<h2 class='title'><a expr:href='data:blog.canonicalHomepageUrl'><data:title/></a></h2>
</b:if>
</b:includable>
</b:widget>
Source
http://blogr-amp.blogspot.co.id/2017/02/amp-html-header-gadgetwidget-for.html