Source

File: wp-includes/widgets/class-wp-widget-text.php

public function inject_video_max_width_style( $matches ) {
		$html = $matches[0];
		$html = preg_replace( '/\sheight="\d+"/', '', $html );
		$html = preg_replace( '/\swidth="\d+"/', '', $html );
		$html = preg_replace( '/(?<=width:)\s*\d+px(?=;?)/', '100%', $html );
		return $html;
	}