The reference to entity "foo" must end with the ';' delimiter

I have Google checkout sandbox generated HTML code which works fine in HTML page. When I put the same code in XHTML page, it throws the below exception:

> the reference to entity "w" must end with the ';' delimiter

It's referring the request parameter w in the URL in the below src attribute:

<input type="image" name="Google Checkout" alt="Fast checkout through Google"
    src="http://sandbox.google.com/checkout/buttons/checkout.gif?merchant_id=211512493599623&w=180&h=46&style=white&variant=text&loc=en_US"
    height="46" width="180" />

How is this caused and how can I solve it?

The ampersand & is a special character in HTML and XML. If you want to use it as a normal character, you have to encode it correctly. Write & instead of &:

src="...9623&w=180&h=46&style=white&variant=text&loc=en_US"

& denotes the start of an encoded entity, such as < for <, or & for &. In your case the parser tries to interpret &w as an entity. But entities are always terminated by an ;, thus if the ; is missing you get the error message.

This worked on my side, when using search iq's installation code on my Blogger blog's HTML file:

<script type="text/javascript">
(function () {
    window.siqConfig = {
        engineKey: "6e14b3aacb2b93b658f8729adec0c030",
        forceLoadSettings: false        // change false to true if search box on your site is adding dynamically
    };
    window.siqConfig.baseUrl = "//pub.searchiq.co/";
    var script = document.createElement("SCRIPT");
    script.src = window.siqConfig.baseUrl + '/js/container/siq-container-2.js?cb=' + (Math.floor(Math.random()*999999)) + '&engineKey=' + siqConfig.engineKey;
    script.id = "siq-container";
    document.getElementsByTagName("HEAD")[0].appendChild(script);
})();

Gave me error here: &engineKey, after adding & replacing the & I was able to save my HTML file.