Summary
Line length is the number of characters that appear in one line of text. Line length is an important concept in typography and web design, as it affects how readers process the information they are reading. Too few characters per line will lead to sentences and paragraphs unnecessarily wrapping to new lines and staggering the flow of the text, as well as causing the readers' eyes to have to move back and forth more frequently, tiring them out in the process. Too many characters may also cause eye strain, as well as cause the reader to lose focus, lose track of which line they are reading, and may make it harder to retain the information. Most readers aren't actively aware of the issue of improper line length, and are likely to stop reading something with an improper line length because they find it hard to follow.
Different sources provide different ranges for proper line length, but generally it is accepted that 45-90 characters is an acceptable range. It can also be thought of as between 2 and 3 alphabets (proper line length will allow you to write the entire alphabet between 2 and 3 times). On mobile devices, an ideal line length is closer to 25 characters (or approximately 1 alphabet).
Solution
To mitigate the issue of line length, CSS can be used to set a font size that changes with the viewport width. This can be done using media queries, “rem” units, and “em” units. Media queries can allow you to adjust the size of the display based on the size of the viewport, and em units allow you to adjust the size of the font based on the size of the parent element.
Using em instead of px allows the font size and container size to scale with the size of the viewport. Media queries should be used to adjust the max width of the container on larger screens so the line length does not grow too large. 16 px is the standard font size, but this can and should be adjusted depending on the size of the device (smaller devices can rely on a smaller font size, considering mobile users will look more closely at their phone than they would a desktop computer or laptop).