Bootstrap 4 重置了所有的 HTML 标题 (h1 到 h6 )的样式

h1 Bootstrap 标题 (2.5rem = 40px)

h2 Bootstrap 标题 (2rem = 32px)

h3 Bootstrap 标题 (1.75rem = 28px)

h4 Bootstrap 标题 (1.5rem = 24px)

h5 Bootstrap 标题 (1.25rem = 20px)

h6 Bootstrap 标题 (1rem = 16px)