为了浏览器兼容、统一、规范性的CSS群定义

body, div, address, blockquote, iframe, ul, ol, dl, dt, dd,
h1, h2, h3, h4, h5, h6, p, pre, table, caption, th, td,
form, legend, fieldset, input, button, select, textarea {
    margin: 0;
    padding: 0;
    font-weight: normal;
    font-style: normal;
    font-size: 100%;
    font-family: inherit;
}
ol, ul {
    list-style: none;
}
fieldset, img {
    border: 0;
}
:focus {
    outline: 0;
}
table {
    border-collapse: collapse;
    border-spacing: 0;
}
caption, th {
    text-align: left;
}