A couple of minor nits with items in the Tools menu (Sigil 0.9.3, Win10 x64):
The code I use for cover images is:
<div class="svg_wrapper">
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 1000 1500" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink"><image height="1500" width="1000" xlink:href="../Images/cover.jpg"></image>
</svg>
</div>
The file cover.jpg exists in the Images directory and displays quite happily in book view using the code above. However when I click on Delete Unused Media Files, I get cover.jpg listed as an unused file. Oddly, cover.jpeg which is unused is not flagged as unused.
When I click on Well-Formed Check EPUB, it complains about the code used to display the cover. Changing <image height="1500" width="1000" xlink:href="../Images/cover.jpg"></image> to <image height="1500" width="1000" xlink:href="../Images/cover.jpg" /> removes the error. I probably should use the second form but it's a bit of boilerplate I've used for quite a while and I've had no other epub check complain about it.