Side Effects from HTML to HTML conversion

["S. Manning" <scriptor-aFO/2INALiozYggVrLCuDg@public.gmane.org>]

I seem to still be getting side effects when I take HTML as input and 
output to HTML (so all I use pandoc for is to take some variables and 
wrap the contents in header and footer code with the variables inserted 
in the appropriate places).  Passages like the following in the input:

<figure>
	<a href="/images/2021/04/acme-widgets.jpg">
	<img src="/images/2021/04/acme-widgets.jpg" alt="a mysterious machine 
sticking out of a cardboard shipping box" />
	<figcaption aria-hidden="true">One of this proud company's most famous 
products, the type 37 widget ...</figcaption>
	</a>
</figure>

become like so in the output:

<figure>
<img src="/images/2021/04/acme-widgets.jpg" alt="One of this proud 
company's most famous products, the type 37 widget ..." /><figcaption 
aria-hidden="true">One of this proud company's most famous products, the 
type 37 widget ...</figcaption>
</figure>

I lose the <a> tag and I lose the contents of the alt attribute (good 
alt text is not the same as a good caption!  The caption tells you how 
to interpret the picture, the alt text tells you what the picture would 
be if you could see it).  Are there any ways of avoiding these side 
effects?

If any of you can suggest a more appropriate tool than pandoc for my use 
case (take a HTML fragment and some metadata, wrap the fragment in 
header and footer text with some values inserted from the metadata to 
create a valid HTML file) I will consider it.