Development trends:
html (Hypertext markup language)-xhtml (Extensibility Hypertext markup language)-xml (Extensibility markup language);
html:
1. Insensitive to upper and lower case;
2. Labels do not have to appear in pairs;
3.<br>;
xhtml:
1. Be sensitive to upper and lowercase;
2. The tags must appear in pairs, and if there is a start tag, there must be an end tag;
3. The attribute value must be in quotes;
4. Not supported attribute minimization:
eg: Correct: <input checked='checked'>
Error: <input checked>
5. If the name attribute is not approved for use, it will be eliminated in the future;
6. The empty element must also end the tag: such as: <br/>, <hr/> (horizontal dividing line)
xml:
html will eventually develop into xml. xhtml is a transition for html to develop xml, and the characteristics of xhtml are also suitable for xml;