The Casper theme by default has an excerpt with all HTML tags / formatting removed. You can change this in your theme by editing the /content/themes/mytheme/partials/loop.hbs file of your theme.
In the loop.hbs file you will see:
{{excerpt words="26"}}
You will need to change the word excerpt to content. The new code will be:
{{content words="26"}}
If you are wanting to change the length (amount of words) of the excerpt please see here.