Combine calc() with attr() in CSS

There appears to be a way around it using vars

.content{
    --x: 1;
    width: calc(100px * var(--x));
    background: #f00;
}

[data-x="1"] { --x: 1; }
[data-x="2"] { --x: 2; }
[data-x="3"] { --x: 3; }

/*doesn't look like this works unfortunately
[data-x] { --x: attr(data-x); }
seems to set all the widths to some really large number*/

The commented out section would have been perfect, and this may be the very same reason your idea didn't work, but it seems css doesn't perform the nice automatic casting that you might be used to in javascript ('2' * 3 //=6).
attr() returns a string, not a number, and this can be seen by adding .content:after { content:var(--x) }; nothing gets printed, --x is a number, content accepts strings.

If there is some css function to cast I feel like that would be the key to this problem.

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Looks like casting _{(well, interpreting)} will be a thing in CSS4, and it'll be as simple as

.content{
    width: calc(100px * attr(data-x number, 1));
    background: #f00;
}

To date, no browsers support even this experimental spec, but I'll update when it does.

Right now attr() is not supported by default in any major browser for any attributes other then "content". Read more about it here: https://developer.mozilla.org/en-US/docs/Web/CSS/attr