If a block returns a renderable array and the array has direct attibutes they will be "stolen" from the original content and applied to the block itself.
For example a if a block returns something like this:
<?php
array(
'#attributes' => array(
'class' => array(
'foo-bar'
),
'data-test' => 'This is just a test'
),
'#markup' => 'This is block content'
);
?>The rendered HTML will be:
<div id="block-foobartest" class="foo-bar" data-test="This is just a test">
This is block content.
</div>instead of:
<div id="block-foobartest">
<div class="content foo-bar" data-test="This is just a test">
This is block content.
</div>
</div>This is especially bad for blocks that represent forms since none of the form attributes is preserved.