Responsive landing page using css in (what I thought) was ok

I'm happy with the results, but would be honored to receive your opinion.

I met a few problems along the way, and have some doubts to know if that was ok.

• I was unable to place the <footer> at the very bottom of the screen it kept going up whenever the view got more mobile, to the point of overlapping with the main elements. I did that because when I left it untouched some background appeared on it (not sure from where and made a horrible white line at the bottom of the elements, rendering my shadow-box useless).

• I added an <a> inside the <button> to place the link to the login there, is that alright? Should it be an href inside the <button>?

• Please (is possible) let me know if the CSS is easy to read as this is the first time I share a project with someone and I really don't know if I should be doing something else to make it easier for people to navigate my docs.

Thanks in advance!!