Maybe I didn’t explain this clearly enough: Why does the image.php template file in the Twenty Twelve WP theme contain this invalid HTML 5?
<article id="post-<?php the_ID(); ?>" <?php post_class( 'image-attachment' ); ?>>
<header class="entry-header">
<h1 class="entry-title"><?php the_title(); ?></h1>
<footer class="entry-meta">
<?php
$metadata = wp_get_attachment_metadata();
printf( __( '<span class="meta-prep meta-prep-entry-date">Published </span> <span class="entry-date"><time class="entry-date" datetime="%1$s">%2$s</time></span> at <a href="%3$s" title="Link to full-size image">%4$s × %5$s</a> in <a href="%6$s" title="Return to %7$s" rel="gallery">%8$s</a>.', 'twentytwelve' ),
esc_attr( get_the_date( 'c' ) ),
esc_html( get_the_date() ),
esc_url( wp_get_attachment_url() ),
$metadata['width'],
$metadata['height'],
esc_url( get_permalink( $post->post_parent ) ),
esc_attr( strip_tags( get_the_title( $post->post_parent ) ) ),
get_the_title( $post->post_parent )
);
?>
<?php edit_post_link( __( 'Edit', 'twentytwelve' ), '<span class="edit-link">', '</span>' ); ?>
</footer><!-- .entry-meta -->
<nav id="image-navigation" class="navigation" role="navigation">
<span class="previous-image"><?php previous_image_link( false, __( '← Previous', 'twentytwelve' ) ); ?></span>
<span class="next-image"><?php next_image_link( false, __( 'Next →', 'twentytwelve' ) ); ?></span>
</nav><!-- #image-navigation -->
</header><!-- .entry-header -->
A <footer>
element inside of (child of) a <header>
element. This is wrong and does not validate. Is there some specific reason for doing this?
That was a good joke though about “familiar with HTML5…” Haw haw haw.