- Introduced in HTML 1.0 or 2.0
- Introduced in HTML 3.2
- Introduced in HTML 4.0

- First supported in Netscape Navigator 1 or 2
- First supported in Netscape Navigator 3
- First supported in Netscape Navigator 4

- First supported in Microsoft Internet Explorer 1 or 2
- First supported in Microsoft Internet Explorer 3
- First supported in Microsoft Internet Explorer 4