If I understand your question correctly, you have html in an Excel cell and you want to make parts of that html dynamic by referencing content in other cells of the workbook.
I assume that in your example you want to make the imagesource and the Product Name dynamic.
You can copy and paste the html into the Excel formula editor. You can increase its height, so you see more than one line at a time. The formula editor can handle line breaks.
If you want to build a string that contains double quotes, you will need to use two double quotes if the quote is inside the string and three double quotes in a row if it is at the beginning or end of a string. You can use the ampersand to concatenate strings and cell references.
With your specific example above, the formula in Excel would read somewhere along these lines (replace Sheet2!A2 etc. with the cell that holds your data. Arrange that data in a table with a row for each product, then you can copy this formula down to get the desired result.
="<div id=""productimage"" style=""float:left;width:380px;"">
<img alt=""alternate"" src="""&Sheet2!A2&""" />
<div id=""productspecs"" style=""float:left;padding-left:25px;"">
Turn on "Wrap Text" in the cell format, otherwise you will see it all in one line of code. The screenshot below uses two rows of data with different texts for image source and product name in sheet 2.
EDIT: I tried to post this in a comment, but the double and triple quotes don't make it and get replaced with just one quote.
="First String"&A1&"Next String"
If the string has quotes inside, double them
="He said "Please" but nobody heard him"&A1&"next string"
If the string has quotes at the beginning of the string, then you need the opening quote for the string and the double quote for the quote inside the string. Likewise for quotes at the end of the string: duplicate the quote in the string and then add the closing quote.
="""Please" - he said"&A1&"and she answered "OK."""
I'm having trouble with this. Here's the Excel box code: <div id="productimage" style="float:left;"> <img alt=""""!A1&"""" src=""""!B1&"""" /> </div> <div id="productspecs" style="float:right;padding-left:25px;"> <h2><strong>"""!A1&"""</strong></h2> It doesn't seem to be picking up the values from A1 and B1 .. I'm not seeing it get filled into C1 (Where this formula is used).
What is the ! doing in front of the A1 and B1? Do you know how a cell reference works? The ! separates a sheet name from a cell address. If there is no sheet name there is no ! sign. So you would use Sheet2!A1 if the cell is on Sheet2 and the formula is on a different sheet. If the referenced cell is on the same sheet, just use A1.
Also, you managed to delete some of the & signs that concatenate the different strings. Please see my edit in the post, since the comment won't take it properly.