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$(document).ready(function () {
    $("#gdRows td").each(function () {
        var cellText = $(this).text();
        if ($.isNumeric(cellText)) {
            var iNum = parseInt(cellText);
            if (iNum < 0) $(this).addClass('negative');
        }
    });
});

<table id="gdRows" style="color:Black;background-color:LightGoldenrodYellow;border-color:Tan;border-width:1px;border-style:solid;font-family:Arial;font-size:Small;width:80%;border-collapse:collapse;">
    <tr align="left" style="background-color:Tan;font-weight:bold;">
        <th scope="col">ID</th>
        <th scope="col">Product Name</th>
        <th scope="col">Quantity</th>
        <th scope="col">Profit</th>
    </tr>
    <tr align="left">
        <td>1</td>
        <td>Mouse</td>
        <td>10</td>
        <td>100</td>
    </tr>
    <tr align="left" style="background-color:PaleGoldenrod;">
        <td>2</td>
        <td>Speaker</td>
        <td>15</td>
        <td>-990</td>
    </tr>
    <tr align="left">
        <td>3</td>
        <td>Hard Drive</td>
        <td>-35</td>
        <td>3990</td>
    </tr>
    <tr align="left" style="background-color:PaleGoldenrod;">
        <td>4</td>
        <td>RAM</td>
        <td>22</td>
        <td>399</td>
    </tr>
    <tr align="left">
        <td>5</td>
        <td>Wireless Keyboard</td>
        <td>10</td>
        <td>3500</td>
    </tr>
    <tr align="left" style="background-color:PaleGoldenrod;">
        <td>6</td>
        <td>Wi-Fi Router</td>
        <td>-1</td>
        <td>3990</td>
    </tr>
    <tr align="left">
        <td>7</td>
        <td>LCD</td>
        <td>62</td>
        <td>3000</td>
    </tr>
    <tr align="left" style="background-color:PaleGoldenrod;">
        <td>8</td>
        <td>Intel Processor</td>
        <td>-5</td>
        <td>7000</td>
    </tr>
    <tr align="left">
        <td>9</td>
        <td>Monitor</td>
        <td>25</td>
        <td>1990</td>
    </tr>
    <tr align="left" style="background-color:PaleGoldenrod;">
        <td>10</td>
        <td>Keyboard</td>
        <td>20</td>
        <td>-350</td>
    </tr>
</table>