In HTML5, the <head> element can be omitted. The zend-developer-tools css requires it.

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In HTML5, the <head> element can be omitted. The following code will validate as HTML5:

<!DOCTYPE html>
<html>
<title>Title of the document</title>

<body>
<h1>This is a heading</h1>
<p>This is a paragraph.</p>
</body>

</html>

However, ZendDeveloperTools\Listener->injectToolbar() requires it to work

$injected = preg_replace('/<\/head>/i', $style . "\n</head>", $injected, 1);

Styles should be displayed in the html document even if there is no <head> html element when the document is html 5 document.

html document does not include zend-developer-tools required css, if <head> element is missing from the layout.

laminas / laminas-developer-tools