Apply style to cells of first row

Use tr:first-child to take the first tr:

.category_table tr:first-child td {
    vertical-align: top;
}

If you have nested tables, and you don’t want to apply styles to the inner rows, add some child selectors so only the top-level tds in the first top-level tr get the styles:

.category_table > tbody > tr:first-child > td {
    vertical-align: top;
}