Is <tr> tag optional?

Question

<table>
  <td>cell1</td>
  <td>cell2</td>
</table>

I found out that this tags work on ie8, firefox 11 and chrome, but not sure whether this is valid by standards and work on all major browsers.

Answer 1

All browsers accept "quirks" in HTML layout and will render a "best guess" based on what they find. But in the HTML specifications, a <tr> tag is DEFINITELY required — regardless of whether it works or not, you should always use valid HTML!

Check early, check often!

http://validator.w3.org/

Answer 2

<tr> tag is of course required with <table>, it specifies the begining and ending of the particular row, so it can't be neglected.