I just wanted to use the details/summary tag on my webpage. Using the summary tag, I'm able to define what my caption looks like. BUT, as soon as I want to edit the page in the visual editor, wordpress removes the summary tag and the standard 'Details' is shown. Is that a bug, or am I doing anything wrong?
WordPress filters the markup used in post content as a security measure. This stops people inserting malicious javascript inside comments or posts.
On a non-multisite install, administrators sometimes have the unfiltered html capability allowing them to bypass this, but this is a dangerous feature
In particular, the wp_kses_post function replicates this functionality and maintains the whitelist WP uses.
A more compatible approach would be to use a shortcode. However, if you can do what you need to do within the WP UI, then you should do that, and focus on changing the default markup used via actions/filters, rather than inserting your own by hand
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