What is doctype and what happens if doctype is not declared in a document?

Related questions

1. Where is my website HTML?
2. Is html5 compatible with older browsers?
3. What DTD means?
4. What is the difference between doctype HTML and HTML?
5. How can we create doctype in html5?
6. Does html5 need doctype?
7. What is the html5 stack?
8. How do you know which HTML version I am using?
9. What does doctype mean?
10. What is doctype XML?

Tags

1. #documents
2. #language
3. #require
4. #correctly
5. #document
6. #therefore
7. #reference
8. #html5
9. #render
10. #knows
11. #browsers
12. #declaration
13. #expect
14. #markup
15. #doctype
16. #content
17. #rules
18. #browser
19. #based
20. #specifies
21. #always