Have a look at https://github.com/ezsystems/ezpublish/blob/master/settings/content.ini#L143
that's it. you can add your "available classes" in one of your content.ini.append.php files. clear your caches and 'large' should appear in the drop-down
again in the same file have a look at https://github.com/ezsystems/ezpublish/blob/master/settings/content.ini#L112
you can add a custom tag to that list.
After this, you need to add a template for the output part. this template should be in one your design/[whatever_design_used_by_your_site]/templates/content/datatype/view/ezxmltags.
I've followed what you suggested and I can select the 'hr' option from within the Custom Tag popup but it doesn't display the <hr> when I browse the site.
I added hr.tpl to the folder /extension/my_extension/design/ezwebin/override/templates/datatype/ezxmltext/hr.tpl
Seems like a lot of work to display a simple <hr> tag.
I've followed what you suggested and I can select the 'hr' option from within the Custom Tag popup but it doesn't display the <hr> when I browse the site.
I added hr.tpl to the folder /extension/my_extension/design/ezwebin/override/templates/datatype/ezxmltext/hr.tpl
Seems like a lot of work to display a simple <hr> tag.
Jus a few tips to check:
you must add the definition of your custom tag to override/content.ini... (or to all siteaccesses you're using)
try clearing the cache
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