Edit in JSFiddle

var gulp = require('gulp'),
    minifycss = require('gulp-minify-css'),
    cmq = require('gulp-combine-media-queries'),
    uncss = require('gulp-uncss'),
    rename = require("gulp-rename"),
    request = require("request"),
    fs = require("fs");

var css = './css/style.css';
var url = 'your-url-here';

gulp.task('css', function(){
    
    request(url).pipe(fs.createWriteStream('page.html'));

    gulp.src(css)
        .pipe(uncss({
            html: ['page.html'],
            ignore: ['.hovering'],
            timeout: 1000
        }))
        .pipe(cmq({log: true}))
        .pipe(minifycss({
            keepSpecialComments: 0,
            noAdvanced: true
        }))
        .pipe(rename("style.min.css"))
        .pipe(gulp.dest('css'));
});

gulp.task('default', function(){
    gulp.start('css');
});