[html] How to center a table of the screen (vertically and horizontally)

Horizontal centering is easy. You just need to set both margins to "auto":

table {
  margin-left: auto;
  margin-right: auto;
}

Vertical centering usually is achieved by setting the parent element display type to table-cell and using vertical-align property. Assuming you have a <div class="wrapper"> around your table:

.wrapper {
  display: table-cell;
  vertical-align: middle;
}

More detailed information may be found on http://www.w3.org/Style/Examples/007/center

If you need support for older versions of Internet Explorer (I do not know what works in what version of this strange and rarely used browser ;-) ) then you may want to search the web for more information, like: http://www.jakpsatweb.cz/css/css-vertical-center-solution.html (just a first hit, which seems to mention IE)