Web banners: customisable close button (HTML)

Example in CSS:

.close {
        	    width: 40px;
    			height: 40px;
    			border: 0;
    			display: flex;
    			display: flex;
    			align-items: center;
    			justify-content: center;
    			position: fixed;
    			right: 0;
    			top: 0;
    			background: transparent;
    			cursor: pointer;
    			z-index: 1;
        	}
Markup (html):
<button class="close" onclick="window.parent.MeiroEvents.closeWebBanner()">