Polyfill "style once" hopefully standardised soon :)

style-once.js

const selector = 'link[rel="stylesheet"], style';

onElement(selector, function(el){ // NOT IMPLEMENTED IN THIS SCRIPT
    // checks all styleSheets and importRules, todo store urls global and check all if used url found
    // console.log(el.sheet)
    checkAllSheets()
});

function checkAllSheets(){
    const urls = {};
    for (let sheet of document.styleSheets) checkSheet(sheet);

    function checkSheet(sheet){
        try { // firefox fails sometimes if we access sheet.cssRules immediate after found it
            checkImportRules(sheet.cssRules);
        } catch (e) {
            //console.log('fail',sheet, e) //
        }
        if (!sheet.href) return;
        const hasAttribute = sheet.ownerNode && sheet.ownerNode.hasAttribute('once');
        const hasMediaOnce = medialist_has(sheet.media, 'once');
        if (hasAttribute || hasMediaOnce) {
            sheet.disabled = !!urls[sheet.href];
            urls[sheet.href] = 1;
        }

    }
    function checkImportRules(rules){
        for (let rule of rules) {
            if (rule instanceof CSSImportRule) checkSheet(rule.styleSheet);
        }
    }
}

function medialist_has(mediaList, wanted){
    for (let medium of mediaList) {
        if (medium === wanted) return true;
    }
}

Handle once in style-links

<link rel=stylesheet once href="...">

And css imports

 @import url("bluish.css") once;