Default html header

def _header(self):
        """
        Default html header
        """
        html = """
		<!DOCTYPE html>
		<html lang="en">
		<head>
			<meta charset="utf-8">
			<title>Report</title>
		"""
        if "bokeh" in self.report_engines:
            html += self.bokeh_header_()
        if "altair" in self.report_engines:
            html += self.altair_header_()
        if "chartjs" in self.report_engines:
            html += self.chartjs_header_()
        html += """
		</head>
		<body>
		"""
        return html