<p>Using OpenXML SDK, I want to insert basic HTML snippets into a Word document.</p>

<p>How would you do this:</p>

<ul>
<li>Manipulating XML directly ?</li>
<li>Using an XSLT ?</li>
<li>using AltChunk ?</li>
</ul>

<p>Moreover, C# or VB examples are more than welcome :)</p>

I'm not sure, what you actually would like to achieve. The OpenXML documents have an own html-like (WordprocessingML) notation for the formatting elements (like paragraph, bold text, etc.). If you would like to add some text to a doc, with basic formatting, than I rather suggest to use the OpenXML syntax and format the inserted text with that.
If you have a html snippet, that you must include into the doc as it is, you can use the "external content" feature of OpenXML. With external content, you can include the HTML document to the package, and create a reference (altChunk) in the doc in the position, where you want to include this. The disadvantage of this solution, that not all tools will support (or support properly) the generated document, therefore I don't recommend this solution, unless you really cannot change the HTML source.
How to include any content (the wordml) to a openxml word doc is an independent question IMHO, and the answer depends very much on how complex modifications you want to apply, and how big the document is. For a simple document, I would simply read out the document part from the package, obtain it's stream and load it to an XmlDocument. You can insert additional content to the XmlDocument quite easily, and then save it back to the package. If the document is big, or you need complex modifications in multiple places, XSLT is a good option.