为什么没报错啊？麻烦老师看一下

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE teaching-plan SYSTEM "plan.dtd">
<!-- 教学计划 -->
<teaching-plan>
<!-- 课程 -->
 <course>
  <course-name>大学英语</course-name>
  <class-hour>36</class-hour>
  <exam-form>考试</exam-form>
 </course>
 <course>
  <course>高等数学</course>
  <class-hour1>70</class-hour1>
  <exam-form>考试</exam-form>
 </course>
 <course>
  <course>计算机应用基础</course>
  <class-hour>108</class-hour>
  <exam-form>上机考试</exam-form>
 </course>
</teaching-plan>

<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT teaching-plan (course+)>
<!ELEMENT course (course-name,class-hour,exam-from)>
<!ATTLIST course-name (#PCDATA)>
<!ATTLIST class-hour (#PCDATA)>
<!ATTLIST exam-from (#PCDATA)>