[drivers99]

It's a browser setting. For example, I'm using Firefox 41 on Windows 7 and in Options->Content->Fonts&Colors/Advanced... it has "Times New Roman", "Arial", and "Courier New" as the defaults for Serif, Sans-serif, and Monospace respectively.

In Chrome, it's under Settings->+Show advanced settings->Web content/Customize fonts... (Times New Roman, Arial, Consolas on mine.)

In IE 11 under Tools (gear icon)->General->Appearance/Fonts it has "Webpage font" and "Plain text font" (Times New Roman and Courier New on mine). It's weird that they don't let you pick the default sans-serif font.