How to flim strip vertical

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.filmstrip {
  --background: rgba(0, 0, 0, .35);
  --size: 10px;
  background-image:
    linear-gradient(to right, var(--background) var(--size), transparent var(--size)),
    linear-gradient(to bottom, var(--background) var(--size), transparent var(--size)),
    linear-gradient(to right, var(--background) var(--size), transparent var(--size)),
    linear-gradient(to bottom, var(--background) var(--size), transparent var(--size)),
    linear-gradient(to bottom, transparent var(--size), var(--background) var(--size));
  background-size: calc(var(--size) * 2) var(--size), calc(var(--size) * 2) var(--size), calc(var(--size) * 2) var(--size), calc(var(--size) * 2) var(--size), 100% calc(100% - var(--size) * 3);
  background-repeat: repeat-x;
  background-position: 0 var(--size), top left, 0 calc(100% - var(--size)), bottom left, 0 var(--size);
  padding: calc(var(--size) * 3) calc(var(--size) * 2.5);
  box-sizing: border-box;
}

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<div class="filmstrip"><h1>Stuff in the Film Strip</h1></div>

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I did repair code

background-repeat: repeat-y;

But I don't suscess

I've fixed a lot hope you guys can help me
can you help me please

I would consider one conic-gradient

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.filmstrip {
  --background: rgba(0, 0, 0, .35);
  --size: 10px;
  
  background:
   conic-gradient(at var(--size),var(--background) 75%,#0000 0)
   0 0/calc(100% - var(--size)) calc(2*var(--size)) padding-box;
   
  border: var(--size) solid var(--background);
  padding: calc(var(--size) * 2.5);
}

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<div class="filmstrip"><h1>Stuff in the Film Strip</h1></div>

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And to have the initial direction we switch a few values

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.filmstrip {
  --background: rgba(0, 0, 0, .35);
  --size: 10px;
  
  background:
   conic-gradient(at 50% var(--size),var(--background) 75%,#0000 0)
   0 0/calc(2*var(--size)) calc(100% - var(--size)) padding-box;

  border: var(--size) solid var(--background);
  padding: calc(var(--size) * 2.5);
}

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<div class="filmstrip"><h1>Stuff in the Film Strip</h1></div>

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