Open W3C HTML validator.
Copy and paste following HTML code to the validator.
<!doctype html><html><head><metacharset="utf-8"><title>I'm very cool title for a cool page</title></head><body><section><button>A</button><button>B</button><button>C</button><button>D</button><buttonhidden>E</button></section><sectionhidden><h1>Hello H1!</h1><h2>Hello H2!</h2><h3>Hello H3!</h3></section><comment><!--I'm a comment--></comment><dialog-block><dialogopen>always open</dialog></dialog-block></body></html>
Answer questions below based on the validator feedback.
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