<wrapper> not working in CSS

[pumpkinpisqui]

Hello there! I am a student who's doing a project in which I have to create a web site with Atom. I'm coding it in HTML and CSS and everything goes like planned but for . When I add wrapper with id
(In this way)

......

and, then, code css

#wrapper{
Width: 90%;
...
}

I have no response. I had the same problem other times (like with :not("...")) and I don't know what's wrong on what I'm doing since, reading what the professor coded, I've made no mistakes. Can someone help me? Thanks in advantage.

[pumpkinpisqui]

Errata corrige

The app doesn't report it but I've written:

Body
Wrapper id="wrapper"
...
/wrapper
/body

With "<>"

[pumpkinpisqui]

Solved