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I would like to build my own version of the card control so that I can control the space inside the card.

I have tried to use the card control but there is a whitespace inside I cannot get rid of.
I want to build a card control of my own wit similar features but mainly to have a heading and a pop up button to appear on mouse over.
The existing card control does not do this.

In the card object, each card item has its own padding and margin. This can be used to control the spacing.
The card also supports animations, transitions and events. So, you have many options to implement advanced functionality.
If you need help with this then please share a demo project and a description what you want to do.

Card demos:
https://www.wysiwygwebbuilder.com/support/wb18tryouts/wb18_card_motioneffects.html
https://www.wysiwygwebbuilder.com/support/wb17tryouts/wb17_portlets.html
https://www.wysiwygwebbuilder.com/support/wb17tryouts/wb17_timeline1.html
https://www.wysiwygwebbuilder.com/support/wb17tryouts/wb17_ecommercecards.html
https://www.wysiwygwebbuilder.com/support/wb17tryouts/wb17_portlets.html
https://www.wysiwygwebbuilder.com/support/wb16tryouts/wb16_listitems.html
https://www.wysiwygwebbuilder.com/support/wb16tryouts/wb16_cards_collapsible.html
https://www.wysiwygwebbuilder.com/support/wb15tryouts/card-demo.html
https://www.wysiwygwebbuilder.com/support/wb15tryouts/card-container.html
https://www.wysiwygwebbuilder.com/support/cards/index.html

You can create your own extensions via Extension Builder (for advanced users).
https://www.wysiwygwebbuilder.com/extension_builder.html