HTML: Replacement for <center>

Question

I always thought that replacing the <center> tag with <div style="text-align:center;"> will get me the same results. Apparently I was wrong.

This is a portion of my HTML: (you can also see it in action in the page I created for this question : http://www.catmoviez.com/ErrorPageSO.aspx

<div style="margin: 0 auto; background-color:red;border:5px solid black;margin-top:5px;width:750px;text-align:center;">
    <span style="width:560px;padding-right:10px;text-align:left;float:left;">
    <h1>Oops... We're sorry.</h1>

    <h3>You've just encountered an unknown error. <br /></h3>
    This site is a work-in-progress, we have already been informed of the error and will do our best to fix it. <br />
    We would be thankful if you could contact us through the appropriate button and elaborate on what caused this error to appear.<br />
    <br />
    <h3>
    You can go back to the <a style="text-decoration:underline;" href="Default.aspx">Home page</a> and continue using Moviez.NET.       
    </h3>
    </span><span style="width:180px;float:left;"><img src="Resources/Images/404.jpg" /></span>
</div>

I want to do 2 things:

1. Get Rid of the <center> tag while keeping the div in the center of the page.
2. Make sure the outer DIVs background color and border affect the inner spans.

UPDATE: Objective 1 is completed. Time for objective #2.

Answer 1

Use margin: 0 auto; on your enclosing <div>

<div style="margin: 0 auto; background-color:red;border:5px solid black;margin-top:5px;width:750px;text-align:center;">
  <span style="width:560px;padding-right:10px;text-align:left;">
  <h1>Oops... We're sorry.</h1>

  <h3>You've just encountered an unknown error. <br /></h3>
  This site is a work-in-progress, we have already been informed of the error and will do our best to fix it. <br />
  We would be thankful if you could contact us through the appropriate button and elaborate on what caused this error to appear.<br />
  <br />
  <h3>
  You can go back to the <a style="text-decoration:underline;" href="Default.aspx">Home page</a> and continue using Moviez.NET.           
  </h3>
  </span><span style="width:180px;"><img src="Resources/Images/404.jpg" /></span>
</div>

See it in action.

Reference: CSS: centering things