Ive looked around the net and not found what Im looking for so thought Id ask here.

When you define a table in html you can specify the height and width of a table data frame. what is this unit of measurement?

ie.
<table>
<tr>
<td width="20">
</td>
</tr>
</table>

this is 20 what? its not pixles. Its not characters. what is it!!!

It's a proprietary width, and it doesn't matter. Use CSS to style the width in pixels or em, or even better, just don't use tables for formatting?

I believe it's percent, but really you should use CSS, then you can specify the units yourself
e.g.
<td style="width: 20px;"></td>


With CSS there are many units available: %, px, em, ex (along with absolute units in: inches, cm: centimeters.. but they are rather annoying to use unless you really need to get something positioned when it's printed.)

It's pixels. You can also define it as <td width="20px">.

Or width="20%" if you'd rather. It'll use 20 percent of the table's width, so if the table is constrained to 100 pixels, that cell will be 20% of 100 or... 20 pixels.

Who said he was using tables for layout...?

Usually, setting the width and height of a table means you're using it for layout. Padding is more compliant for data, as it defines a table-wide value.

What? I always set column widths when displaying tabular data. Either percentages, ems if I know the number of characters i'm expecting (e.g. for formatted dates) or pixels.

Although mostly (always?) it's better to set the column widths in the <th> cells.

What Im making is a user defined HTML page using an interface where you set table and element values. Trying to make it simple as possible and want to label the interface with the units of measurement for sizes.

Um no.

If you're doing proper CSS coding it doesn't matter either way, there's no reason to use the width property in the HTML. It should be in the CSS.

Isn't padding a cell property? Specifically, it defines how much space "pads" the text/media from the border of the cell?

Depends if you're refering to CSS or HTML. For HTML, padding is used for cells in a table but in CSS, padding can be used for almost anything that will accept it.

The one thing that annoys me about CSS is that there is no reliable way to center block items. Text-align: center only works on text, so that's useless for divs.

<div align="center"> works, but is deprecated for Strict doctypes, IIRC, so I try not to use it. (I have no idea why the W3C did that)

I know about margin-left: auto and margin-right: auto; but I've had instances where that didn't work. Is there a better way, or even one guaranteed to work and still be valid CSS/HTML?

In general, the situations where centering using auto margins doesn't work are situations where you shouldn't be centering anyways.

But you should really post an example of your problem.

This is a little untidy, but if you know the width of the item you're centring then you can use negative margins to centre it.

e.g.



The idea is that the div is positioned 50% across the page, then moved back half of it's own width.