What is doctype declaration in html5?

Related questions

1. What is the difference between doctype HTML and HTML?
2. What is SGML and DTD?
3. Is html5 compatible with older browsers?
4. Is HTML 5 case sensitive?
5. Why doctype is used in XML?
6. What is doctype in XML?
7. What is the difference between HTML and html5?
8. What is doctype and what happens if doctype is not declared in a document?
9. How do I know html5?
10. What is doctype >? Is it necessary to use in html5?

Tags

1. #reference
2. #language
3. #elements
4. #other
5. #required
6. #html5
7. #short
8. #obsolete
9. #declaration
10. #therefore
11. #because
12. #before
13. #based
14. #versions
15. #earlier
16. #longer
17. #insensitive
18. #doctype
19. #doctypes
20. #appears