programmatically changing webkit-transformation values in animation rules

Use the CSSOM

var style = document.documentElement.appendChild(document.createElement("style")),
rule = " run {\
    0%   {\
        -webkit-transform: translate3d(0, 0, 0); }\
        transform: translate3d(0, 0, 0); }\
    }\
    100% {\
        -webkit-transform: translate3d(0, " + your_value_here + "px, 0);\
        transform: translate3d(0, " + your_value_here + "px, 0);\
    }\
}";
if (CSSRule.KEYFRAMES_RULE) { // W3C
    style.sheet.insertRule("@keyframes" + rule, 0);
} else if (CSSRule.WEBKIT_KEYFRAMES_RULE) { // WebKit
    style.sheet.insertRule("@-webkit-keyframes" + rule, 0);
}

If you want to modify a keyframe rule in a stylesheet that’s already included, do the following:

var
      stylesheet = document.styleSheets[0] // replace 0 with the number of the stylesheet that you want to modify
    , rules = stylesheet.rules
    , i = rules.length
    , keyframes
    , keyframe
;

while (i--) {
    keyframes = rules.item(i);
    if (
        (
               keyframes.type === keyframes.KEYFRAMES_RULE
            || keyframes.type === keyframes.WEBKIT_KEYFRAMES_RULE
        )
        && keyframes.name === "run"
    ) {
        rules = keyframes.cssRules;
        i = rules.length;
        while (i--) {
            keyframe = rules.item(i);
            if (
                (
                       keyframe.type === keyframe.KEYFRAME_RULE
                    || keyframe.type === keyframe.WEBKIT_KEYFRAME_RULE
                )
                && keyframe.keyText === "100%"
            ) {
                keyframe.style.webkitTransform =
                keyframe.style.transform =
                    "translate3d(0, " + your_value_here + "px, 0)";
                break;
            }
        }
        break;
    }
}

If you don’t know the order but do know the URL of the CSS file, replace document.styleSheets[0] with document.querySelector("link[href="https://stackoverflow.com/questions/5105530/your-css-url.css"]").sheet.

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