element.style.height = '100px';

element.setAttribute('height', 100);
element.setAttribute('height', '100px');

element.setAttribute('style', 'height: 100px !important');

element.style.setProperty('height', '300px', 'important');

element.className = 'blue';
element.className += 'blue fb';

element.style.cssText = 'height: 100px !important';
element.style.cssText += 'height: 100px !important';

function addNewStyle(newStyle) {
var styleElement = document.getElementById('styles_js');
if (!styleElement) {
styleElement = document.createElement('style');
styleElement.type = 'text/css';
styleElement.id = 'styles_js';
document.getElementsByTagName('head')[0].appendChild(styleElement);
}
styleElement.appendChild(document.createTextNode(newStyle));
}
addNewStyle('.box {height: 100px !important;}');

// 在原有样式操作
document.styleSheets[0].addRule('.box', 'height: 100px');
document.styleSheets[0].insertRule('.box {height: 100px}', 0);
// 或者插入新样式时操作
var styleEl = document.createElement('style'),
styleSheet = styleEl.sheet;
styleSheet.addRule('.box', 'height: 100px');
styleSheet.insertRule('.box {height: 100px}', 0);
document.head.appendChild(styleEl);