How to mark only product categories? - microdata

Question

How do I have to mark my website product categories? … only categories …
Maybe with category from Offer?
Something like:
<div itemscope itemtype="http://schema.org/Offer">
<a itemprop="category" href="category1.php">My category 1</a>
</div>

Answer 1

You should not specify category on an a element. The value would be the URI (category1.php), not the content (My category 1).
category expects a value that is either text or another item.
So if you want to provide text, you could use something like:
<div itemscope itemtype="http://schema.org/Offer">
<span itemprop="category">My category 1</span>
</div>
And yes, it is totally fine to use only one property. Schema.org doesn’t define required properties. But consumers (e.g., search engines) might, of course, only consider re-using your data if it fulfils certain of their own requirements.
UPDATE: If you want to provide several categories for the item, it could be something like:
<div itemscope itemtype="http://schema.org/Offer">
<ul>
<li itemprop="category">My category 1</li>
<li itemprop="category">My category 2</li>
<li itemprop="category">My category 3</li>
</ul>
</div>
Don’t rename the category property! Adding 1, 2 etc. to it would be wrong.
(And it it’s a hierarchy of categories, you could use one value and / or >., e.g. Sports > Tennis.)