why do some pokemon cards have yellow border

why do some pokemon cards have yellow border Are these stains on the of my vintage cards? Or just this as part of the design? (Secret wonders Mew) : r/PokemonTCG Pokemon Yellow Border BW Style

Name: why do some pokemon cards have yellow border Are these stains on the of my vintage cards? Or just this as part of the design? (Secret wonders Mew) : r/PokemonTCG Pokemon Yellow Border BW Style
Brand: excellencyproduct.com
SKU: 41752999820
Price: 21.88 USD
Availability: InStock
Rating: 4.4635 (21 reviews)

SKU: 41752999820

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Description

I think Im correct in saying that this is the cheapest (in terms of mana cost) one-sided card draw doubling effect we have now, so although it doesnt double your normal draw step, this is a no-brainer to slot into most blue decks

Vampires werent my first choice to combine with my Pirates, but taking Blue out of the equation really helped me to think more directly about my idea, and I couldnt resist the temptation of giving a tried-and-true tribal deck a fresh coat of paint