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You can learn them at the same time. Nothing bad is going to happen.
css is usable only if you know html. I recommend you to finish html lesson to really know all the bases and like that when the css lesson will give you example you won't be lost. and it's why I did ;)
That's up to you and however you learn. I have the attention span of a little kid, so it's better for me to learn things together instead of focusing on one thing before moving on. It wouldn't hurt to learn CSS and HTML at the same time, as they don't conflict with one another and compliment each other. When I was originally learning HTML, I "learned" CSS by simply looking at a CSS file that was already there and just read it. It's really straight forward, so you'll be able to pick it up easily. However, there is a lot of cool things in CSS3, so I'd learn it properly.
my advice is to finish html first and than start with css :)