If it had neither of these fonts installed, it would show the text as 'Times New Roman'. Almost all computers have this particular font, but if it didn't, it would display the default font as a last resort. Times New Roman is being displayed here as the computer does not have Desdemona or Broadway installed. Sans-serif fonts are generally easier to read on a computer monitor than serif fonts, so they are a good choice for the bulk of the text. Serif fonts work well for headings.
Some fonts work best for printing and others work better for reading on a screen.
For example, Times New Roman is a print font whereas Georgia is a screen font , Arial is a print font whereas Verdana is a screen font . This looks like typewriter text. Here are some examples of fonts with their Mac equivalent. Size 7 is also best avoided as it is larger than Heading 1. The default size is usually size 3.