Source

File: wp-includes/widgets/class-wp-widget-media-video.php

public function inject_video_max_width_style( $html ) {
		$html = preg_replace( '/\sheight="\d+"/', '', $html );
		$html = preg_replace( '/\swidth="\d+"/', '', $html );
		$html = preg_replace( '/(?<=width:)\s*\d+px(?=;?)/', '100%', $html );
		return $html;
	}