The next example not found any node:
from selectolax.lexbor import LexborHTMLParser
html = """
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head>
<body></body>
</html>
"""
tree = LexborHTMLParser(html)
print(tree.css('meta[http-equiv="content-type"]'))
Modest was case insensitive to search attribute values.