A JS script for Derpibooru which orders any tag list of the page by tag count. It's supposed to be used in the developer console (F12 or Ctrl+Shift+I/J).

derpibooru-sort-tag-by-count.js

/* A function to change the page display */
function appendCss(css) {
    var styleElem = document.createElement('style');
    styleElem.textContent = css;
    document.head.appendChild(styleElem);
}

/* A function to extract the tag-count of a tag element */
function extractCount(tagElem) {
    return parseInt(tagElem.innerText.replace(/^.* \((\d+)\) \+$/, "$1"));
}

/* For all tag list of the page */
for (let tagList of document.querySelectorAll(".tag-list")) {
    let tagV = Array.from(tagList.children);
    /* The below line computes the new ordre */
    tagV.sort((tagA, tagB) => extractCount(tagB) - extractCount(tagA));
    /* For all element of the now ordered list */
    for (let el of tagV) {
        /* Move it to the end of the list */
        tagList.appendChild(el);
    }
}

/* The below code changes the display of the tags from horizontal to vertical */
appendCss(`
.tag-list > * {
    display: table;
}
`);