1. Tags are no longer case-sensitive
< ;p>Hello, World!
2. Elements can omit the end tag
(1) Elements of empty element syntax: area, base, br, col , command, embed, hr, img, input, keygen, link, mata, param, source, wbr
(2) The elements of the end tag can be omitted: colgroup, dt, dd, li, optgroup, p, rt, rp , thread, tbody, tfoot, tr, td, th
(3) All tag elements can be omitted: html, head, body, tbody
3. Attributes that allow attribute values to be omitted
4 , allow attribute values to not use quotation marks