Different background colors for each line in the textarea (or editable div?)

Question

I have a textarea for user text input and I want each line to have different background color (from a predefined set of 10 colors, can be repeated if more lines). I understand that I can use CSS expressions like nth-of-type(10n+1) for the tables, static text etc. - but not sure how to implement it for the textarea content.
(attached snippet doesn't really do anything about that, it's just a template to work with)

.my1 {
  background-image:
    repeating-linear-gradient(white, white 30px, gray 31px, white 31px);
  line-height: 31px;
  padding: 8px 10px;
}

<textarea class="my1"></textarea>

Answer 1

Simply define the 10 colors inside your gradient.

Example with 4 colors:

.my1 {
  --l:1.5em; /* height of line  */

  background-image:
    repeating-linear-gradient(
     red    0                calc(var(--l)*1), 
     green  calc(var(--l)*1) calc(var(--l)*2),
     blue   calc(var(--l)*2) calc(var(--l)*3),
     purple calc(var(--l)*3) calc(var(--l)*4));
  line-height: var(--l);
  
  color:#fff;
}

<textarea class="my1"></textarea>

If you want padding, you have to offset the gradient to avoid misalignment. Only an offset from the top is required

.my1 {
  --l:1.5em; /* height of line  */

  background:
    repeating-linear-gradient(
     red    0                calc(var(--l)*1), 
     green  calc(var(--l)*1) calc(var(--l)*2),
     blue   calc(var(--l)*2) calc(var(--l)*3),
     purple calc(var(--l)*3) calc(var(--l)*4)) 0 8px no-repeat,
     yellow;
  line-height: var(--l);
  padding:8px 10px;
  
  color:#fff;
}

<textarea class="my1"></textarea>