如何在ioslides中调整图像或嵌入视频的大小？当使用ioslides_presentation生成slies时，对于简单的.html输出（如下调整宽度和高度）起作用似乎不起作用：
---
title: "test"
author: ""
date: "January 12, 2016"
output: ioslides_presentation
---
## test video
<div align="center">
<iframe width="400" height="150" src="https://www.youtube.com/embed/_LGkbvkCS3I" frameborder="0" allowfullscreen></iframe>
</div>