Help me understand final validation error (xml related)

Hi, I've spent the day validating my site and it's been eye-opening and satisfying to remove errors one by one. However, one problem remains, and I just can't figure it out. I get the error...:

Error Line 1202, Column 8: entity end not allowed in processing instruction

border-color and rbga(...)

Hi,

I use the construct

form#login ol li {
  background-color: rgba(255, 255, 255, .3); /* Valid */
  border-color: rgba(255,255,255,.6); /* Gives an error while validating */
  /* ... */
}

While validating trough w3c validator I get the following error

Value Error : border-color rgba(255,255,255,0.6) is not a border-color value : rgba(255,255,255,0.6) rgba(255,255,255,0.6)

Why? It seems to be a valid colour for me. When I remove the alpha part it validates. I didn't find any hint that alpha channels aren't allowed in border-color in the specifications.

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