I don't think that is possible in an email, nor should it be. There would be major security ramifications.
Agree completely with Bryan and others.
Instead, consider using multiple sections in your email that you can jump to using links and anchors (the 'a' tag). I think that you can emulate the behavior you want by including multiple copies of the text further down in your email. This is a bet messy though, so you could just have sets of anchors that link to each other and allow you to move back in forth between the 'summary' section and the 'expanded' one.
<a href="#section1">Jump to section!</a> <p>A bunch of content</p> <h2 id="section1">An anchor!</h2>
Clicking on the first link will move focus to the sub-section.
Do not depend on this. Any good mail client will not support executable code within an email. Any knowledgeable user will not use a client that does.
The short answer is that scripting is unsupported in emails.
This is hardly surprising, given the obvious security risks involved with a script running inside an application that has all that personal information stored in it.
Keep your emails as straight HTML and CSS, and avoid the hassle. Here is what you can do in html emails: https://www.campaignmonitor.com/guides/coding/technologies/
PS. Didn't see above post's at time of posting. My bad.
Here's what you CAN do:
Then, when the recipient opens the attachment, their web browser will facilitate the dynamic features you've implemented.
you can use html radio/checkbox input with labels and css to achieve the expanding effects you want.
It is best to just use HTML and CSS to style your emails. Maybe you could have a call to action (cta) in your html email that sends the user to a web page with your expanding and collapsing content feature.
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