Hello, congratulations on completing the challenge!
One way to simplify your CSS code would be to use a preprocessor like SASS, which has tools such as mixins and variables to avoid code repetition.
Regarding breakpoints, I suggest using the developer tool to identify the points where the layout begins to break and make adjustments at those points.
Regarding font size, use the REM unit, which utilizes the font settings of the root element. In this element, try setting the font size in percentage, for example:
html{ font-size: 62.5%;}
In this configuration, the font size is set to 10 pixels, so the REM unit will be relative to this value, making it easier for you to use.
Regarding the grid, you wouldn't need to change to flex-box, you could have configured the grid on mobile devices to have only one column. I hope these suggestions have been helpful.
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