Add lazy load to elements within theme

Is it possible to add a class within a theme to make an element lazy load? I’ve got a theme that uses the content filter to run oembed in post headers, but the plugin isn’t catching those iframes and making them lazy load. I wanted to see if there was something I could add within my theme to get it to work. Any suggestions?

https://ww.wp.xz.cn/plugins/a3-lazy-load/