CSS table td width - fixed, not flexible

It is not only the table cell which is growing, the table itself can grow, too. To avoid this you can assign a fixed width to the table which in return forces the cell width to be respected:

table {
  table-layout: fixed;
  width: 120px; /* Important */
}
td {
  width: 30px;
}

(Using overflow: hidden and/or text-overflow: ellipsis is optional but highly recommended for a better visual experience)

So if your situation allows you to assign a fixed width to your table, this solution might be a better alternative to the other given answers (which do work with or without a fixed width)

It's not the prettiest CSS, but I got this to work:

table td {
    width: 30px;
    overflow: hidden;
    display: inline-block;
    white-space: nowrap;
}

Examples, with and without ellipses:

body {
    font-size: 12px;
    font-family: Tahoma, Helvetica, sans-serif;
}

table {
    border: 1px solid #555;
    border-width: 0 0 1px 1px;
}
table td {
    border: 1px solid #555;
    border-width: 1px 1px 0 0;
}

/* What you need: */
table td {
    width: 30px;
    overflow: hidden;
    display: inline-block;
    white-space: nowrap;
}

table.with-ellipsis td {   
    text-overflow: ellipsis;
}

<table cellpadding="2" cellspacing="0">
    <tr>
        <td>first</td><td>second</td><td>third</td><td>forth</td>
    </tr>
    <tr>
        <td>first</td><td>this is really long</td><td>third</td><td>forth</td>
    </tr>
</table>

<br />

<table class="with-ellipsis" cellpadding="2" cellspacing="0">
    <tr>
        <td>first</td><td>second</td><td>third</td><td>forth</td>
    </tr>
    <tr>
        <td>first</td><td>this is really long</td><td>third</td><td>forth</td>
    </tr>
</table>

you also can try to use that:

table {
    table-layout:fixed;
}
table td {
    width: 30px;
    overflow: hidden;
    text-overflow: ellipsis;
}

http://www.w3schools.com/cssref/pr_tab_table-layout.asp