Thymeleaf 提示： Cannot perform conversion to XML from legacy HTML:
SpringBoot 1.5.x 集成Thymeleaf 2.1.x 提示如下错误信息：
[http-nio-9096-exec-1] ERROR org.thymeleaf.TemplateEngine - [THYMELEAF][http-nio-9096-exec-1] Exception processing template "index": Cannot perform conversion to XML from legacy HTML: The nekoHTML library is not in classpath. nekoHTML 1.9.15 or newer is required for processing templates in "LEGACYHTML5" mode [http://nekohtml.sourceforge.net]. Maven spec: "net.sourceforge.nekohtml::nekohtml::1.9.15". IMPORTANT: DO NOT use versions of nekoHTML older than 1.9.15.
2020-08-04 14:09:28.580 [http-nio-9096-exec-1] ERROR org.apache.catalina.core.ContainerBase.[Tomcat].[localhost].[/].[dispatcherServlet] - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.thymeleaf.exceptions.ConfigurationException: Cannot perform conversion to XML from legacy HTML: The nekoHTML library is not in classpath. nekoHTML 1.9.15 or newer is required for processing templates in "LEGACYHTML5" mode [http://nekohtml.sourceforge.net]. Maven spec: "net.sourceforge.nekohtml::nekohtml::1.9.15". IMPORTANT: DO NOT use versions of nekoHTML older than 1.9.15.] with root cause
org.thymeleaf.exceptions.ConfigurationException: Cannot perform conversion to XML from legacy HTML: The nekoHTML library is not in classpath. nekoHTML 1.9.15 or newer is required for processing templates in "LEGACYHTML5" mode [http://nekohtml.sourceforge.net]. Maven spec: "net.sourceforge.nekohtml::nekohtml::1.9.15". IMPORTANT: DO NOT use versions of nekoHTML older than 1.9.15.产生改错误的原因是：thymeleaf 设置HTML5版本：
# 设置HTML 模板
spring.thymeleaf.mode=HTML5很容易出现以下错误：
org.xml.sax.SAXParseException: 元素类型 "input" 必须由匹配的结束标记 "" 终止。为了兼容HTML4 和HTML5 ，修改为HTML 的兼容版本:
# 设置HTML兼容版本
spring.thymeleaf.mode=LEGACYHTML5
使用HTML兼容版本，提示SpringBoot 1.5.x 集成Thymeleaf 2.1.x 提示如下错误信息：
[http-nio-9096-exec-1] ERROR org.thymeleaf.TemplateEngine - [THYMELEAF][http-nio-9096-exec-1] Exception processing template "index": Cannot perform conversion to XML from legacy HTML: The nekoHTML library is not in classpath. nekoHTML 1.9.15 or newer is required for processing templates in "LEGACYHTML5" mode [http://nekohtml.sourceforge.net]. Maven spec: "net.sourceforge.nekohtml::nekohtml::1.9.15". IMPORTANT: DO NOT use versions of nekoHTML older than 1.9.15.
2020-08-04 14:09:28.580 [http-nio-9096-exec-1] ERROR org.apache.catalina.core.ContainerBase.[Tomcat].[localhost].[/].[dispatcherServlet] - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.thymeleaf.exceptions.ConfigurationException: Cannot perform conversion to XML from legacy HTML: The nekoHTML library is not in classpath. nekoHTML 1.9.15 or newer is required for processing templates in "LEGACYHTML5" mode [http://nekohtml.sourceforge.net]. Maven spec: "net.sourceforge.nekohtml::nekohtml::1.9.15". IMPORTANT: DO NOT use versions of nekoHTML older than 1.9.15.] with root cause
org.thymeleaf.exceptions.ConfigurationException: Cannot perform conversion to XML from legacy HTML: The nekoHTML library is not in classpath. nekoHTML 1.9.15 or newer is required for processing templates in "LEGACYHTML5" mode [http://nekohtml.sourceforge.net]. Maven spec: "net.sourceforge.nekohtml::nekohtml::1.9.15". IMPORTANT: DO NOT use versions of nekoHTML older than 1.9.15.

产生改错误的原因是：thymeleaf 设置HTML5版本：
# 设置HTML 模板
spring.thymeleaf.mode=HTML5

很容易出现以下错误：
org.xml.sax.SAXParseException: 元素类型 "input" 必须由匹配的结束标记 "" 终止。

为了兼容HTML4 和HTML5 ，修改为HTML 的兼容版本:
# 设置HTML兼容版本
spring.thymeleaf.mode=LEGACYHTML5

使用HTML兼容版本，提示最开始的错误信息，解决方案是添加nekoHTML 1.9.15 或者更新版本，pom.xml 文件如下：
net.sourceforge.nekohtml
nekohtml
1.9.22