使用HTMLTestRunner生成报告打开为什么不是图形？

生成的报告怎么不是图形import unittest
from uinttest_learn import TestMathFunc
from HTMLTestRunner import HTMLTestRunner

if name == ‘main‘:
suite = unittest.TestSuite()
suite.addTests(unittest.TestLoader().loadTestsFromTestCase(TestMathFunc))

with open('HTMLReport.html', 'w',encoding="utf-8") as f:
    runner = HTMLTestRunner(stream=f,
                            title='MathFunc Test Report',
                            description='generated by HTMLTestRunner.',
                            verbosity=2
                            )
    runner.run(suite)