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HTML prefix attribute
The prefix attribute is added by SEO plugins like Yoast and All in One SEO with Social meta. This results in output such as the following.
Yoast:
<html lang="en-US" prefix="og:" http://ogp_me/ns#="">
All in One SEO Pack with Social Meta
<html lang="en-US prefix=og:" http://ogp_me/ns#="">
I think at least part of the problem is due to the valid space it contains (https://www.w3.org/TR/rdfa-core/#A-prefix) being improperly filtered.
Both plugins add the attribute via add_filter() on language_attributes.
The issue can be replicated by attempting to add the prefix attribute from functions.php with the following:
add_filter( 'language_attributes', 'html_prefix' );
function html_prefix( $atts ) {
$prefix = 'prefix="og: http://ogp.me/ns#"';
$atts .= ' ' . $prefix;
return $atts;
}
