Very impressed to see a mostly functional engine for a card game in the style of Magic: The Gathering! That's not an easy feat, and more complicated than a conventional JRPG battle system. This entry also engaged with the elemental theme. I played a round using the fire deck and won by eliminating enemy summons with direct damage spells while growing my own army of summons past parity.
Development of the game UI seems to be lagging behind the implementation of gameplay features, so additional work on the UI is the obvious biggest priority for future development. The indicators for available mana points should probably use graphical icons or have a mouseover tooltip to make it clear what they represent. The UI for displaying the cards in the player's hand is functional enough for now but would be a good candidate for future improvements - maybe look at Slay the Spire for a good example of how to display and select a hand of cards.
Finally, story and characters are difficult to integrate into a game like this, and there aren't a lot of good examples to use as reference. If the game will be a series of 1v1 duels, then competitive puzzle games like Puyo Pop or Puzzle Fighter might be good examples of how to introduce characters and establish a narrative. There's also an old MtG game from the 90's called Shandalar that was ahead of it's time and has a story and a slate of rival planewalkers to battle against - would recommend checking it out for inspiration if you're not already familiar.