This is an excellent read and addresses issues such as accessible hiding, and content toggles. In regards to focus order, this article brought up a good point about that the visual presentation on a screen might not match the DOM order because of the ability to reorder content through CSS (thanks to floats and flexbox).
Designing forms for mobile can be tricky with small screen sizes. From this article and from personal experience, it is always best to put labels above form elements (ie. input box).
While none of the current browsers support the new CSS grid rules just yet, they will in the near future. You can still learn about it and test it out in Chrome, Firefox or Opera with special flags enabled. The articles provides more details about this.