实体字符 entity

实体字符指需要在页面上显示的特殊字符
写法：&字母; 或 &数字;
常用字符举例：

&    &   U+00026
<    &lt;    U+0003C
>    &gt;    U+0003E
"    &quot;  U+00022
'    &apos;  U+00027
     &nbsp;  U+000A0
–    &ndash; U+02013
—    &mdash; U+02014
©    &copy;  U+000A9
®    &reg;   U+000AE
™    &trade; U+02122
≈    &asymp; U+02248
≠    &ne;    U+02260
£    &pound; U+000A3
€    &euro;  U+020AC
°    &deg;   U+000B0

MDN解释：https://developer.mozilla.org/en-US/docs/Glossary/Character_reference
全部实体：https://html.spec.whatwg.org/multipage/named-characters.html#named-character-references