XML实体 - 我的代码有什么问题？

Question

我正在参加一门XML课程，我必须通读这本名为“INeasysteps”的书。我试图按照教程的一个关于“添加注释和实体”，但是当我把它在浏览器中我不断收到回应：

Blockquote XML Parsing Error: undefined entity Line Number 11, Column 13: Blockquote

这里是我的代码：请让我知道我做错了。仅供参考，有在JEDIT分析器没有错误，但我得到的，当我把我的XML代码在浏览器上面的错误：

<?xml version="1.0" encoding="UTF-8" ?> 

<!-- XML in easy steps - Page 40. --> 

<?xml-stylesheet 
type = "text/css" href = "history.css" ?> 

<!DOCTYPE doc SYSTEM "history.dtd" > 

<doc> 
<para>Both &html; and &xml; are derived from &sgml; which is, in turn, a descendant of the &gml; that was developed in the 1960s by IBM. </para> 
</doc> 

这里是history.dtd文件

<!-- Define the root element. --> 
<!-- May contain one child element called para. --> 
<!ELEMENT doc (para)> 

<!-- Define the child element. --> 
<!-- May contain Parsed Character Data. --> 

<!ELEMENT para (#PCDATA)> 


<!-- Define entity values. --> 
<!-- Common markup language acronyms. --> 
<!ENTITY html 
"HyperText Markup Language (HTML)" > 
<!ENTITY xml 
"eXtensible Markup Language (XML)" > 
<!ENTITY sgml 
"Standard Generalized Markup Language (SGML)" > 
<!ENTITY gml 
"Generalized Markup Language (GML)" > 

Answer 1

大多数浏览器不会加载外部实体。如果你想打开一个浏览器和实体的XML文件来妥善解决，（在DOCTYPE声明[和]之间）添加实体声明到内部子集：

<?xml version="1.0" encoding="UTF-8" ?> 
<!-- XML in easy steps - Page 40. --> 
<?xml-stylesheet type="text/css" href = "history.css" ?> 
<!DOCTYPE doc SYSTEM "history.dtd" [ 
<!ENTITY html "HyperText Markup Language (HTML)" > 
<!ENTITY xml "eXtensible Markup Language (XML)" > 
<!ENTITY sgml "Standard Generalized Markup Language (SGML)" > 
<!ENTITY gml "Generalized Markup Language (GML)" > 
]> 
<doc> 
    <para>Both &html; and &xml; are derived from &sgml; which is, in turn, a 
     descendant of the &gml; that was developed in the 1960s by IBM. </para> 
</doc>