CSS Child vs Descendant selectors

Basics

For controlling "cellpadding" in CSS, you can simply use padding on table cells. E.g. for 10px of "cellpadding":

td { 
    padding: 10px;
}

For "cellspacing", you can apply the border-spacing CSS property to your table. E.g. for 10px of "cellspacing":

table { 
    border-spacing: 10px;
    border-collapse: separate;
}

This property will even allow separate horizontal and vertical spacing, something you couldn't do with old-school "cellspacing".

Issues in IE ≤ 7

This will work in almost all popular browsers except for Internet Explorer up through Internet Explorer 7, where you're almost out of luck. I say "almost" because these browsers still support the border-collapse property, which merges the borders of adjoining table cells. If you're trying to eliminate cellspacing (that is, cellspacing="0") then border-collapse:collapse should have the same effect: no space between table cells. This support is buggy, though, as it does not override an existing cellspacing HTML attribute on the table element.

In short: for non-Internet Explorer 5-7 browsers, border-spacing handles you. For Internet Explorer, if your situation is just right (you want 0 cellspacing and your table doesn't have it defined already), you can use border-collapse:collapse.

table { 
    border-spacing: 0;
    border-collapse: collapse;
}

Note: For a great overview of CSS properties that one can apply to tables and for which browsers, see this fantastic Quirksmode page.

You can check directly at the CSS grammar.

Basically¹, a name must begin with an underscore (_), a hyphen (-), or a letter(a–z), followed by any number of hyphens, underscores, letters, or numbers. There is a catch: if the first character is a hyphen, the second character must² be a letter or underscore, and the name must be at least 2 characters long.

-?[_a-zA-Z]+[_a-zA-Z0-9-]*

In short, the previous rule translates to the following, extracted from the W3C spec.:

In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, or a hyphen followed by a digit. Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier "B&W?" may be written as "B&W?" or "B\26 W\3F".

Identifiers beginning with a hyphen or underscore are typically reserved for browser-specific extensions, as in -moz-opacity.

¹ It's all made a bit more complicated by the inclusion of escaped unicode characters (that no one really uses).

² Note that, according to the grammar I linked, a rule starting with TWO hyphens, e.g. --indent1, is invalid. However, I'm pretty sure I've seen this in practice.