WEB RULES
Low contrast fonts
Low contrast means a darker font on a dark background or a lighter font on a light background. The readability is impacted by such choices and it could also lead to eye strain. Usually you will want your website content to be easy to read. Nearly 9% of Americans are visually impaired.
Background images
There are a number of potential problems to avoid when choosing background images in contemporary web design. Since layouts shift responsively from phone screens to tablets and desktop browsers, the background imagery must work in a variety of layouts. Low contrast imagery can make it easier to overlay text in a legible way. Resolution is also critical because large image files may load slowly on mobile devices.
Small fonts
Eyes over 40, and under 40 for many people, have difficulty reading fonts that are smallish -- what would be called "10 point" in the print world. On screens, the size of fonts are based on pixel density. ("Retina" screens are high resolution, typical desktop monitors are lower resolution.) Each browser has a default body text font size ("baseline font size") that takes into account the resolution of the display. The size of this body text font is 1em. 2em is headline big, 0.7em is practically unreadable for the majority of the public. Take care not to assume that because your 20-something eyes may be able to read small fonts that others will be able to see them clearly.
Color
Color is often the most obvious thing about a design. Color is also capable of creating strong reactions among people, who consciously and subconsciously apply certain meanings or emotions to different colors (this is also influenced by culture, as many colors mean different things in different cultures). Color theory is an important aspect of design, and something designers should know how colors work together to create a mood or feel in a design. Sites like paletton.com and color.adobe.com can help to establish good color choices.
Too Many Fonts
This can make your designs look slapped together. As a general rule, pick a font for the main body text, and another for titles, and stick with them. With practice you may wish to branch out a bit more, but unless you really want to draw attention to the variety of type faces, it is advisable to be conservative. Using a variety of weights within a single font family can provide variety without distraction.