Drop shadow html...

[YinYang]

Hello, I am currently developing a website, and have run into a small problem. I have no idea how to code a drop shadow, How would I go about doing this? should I use CSS or HTML?

here is an example the drop shadow I need to code.

where the dark gray pattern is the background of the website, and to the right is the main website container. I really need help with this ASAP.

Thanks in advance.

[LucasZ21]

I don't think you can do a "dropshadow" with strictly HTML.

It might be possible with CSS but, i'm still learning.