Custom development for HTML5 ads involves creating interactive, multimedia ads using HTML5, CSS, and JavaScript. These ads can include animations, videos, and other interactive elements that are not possible with traditional image-based ads. Custom development for email ads typically involves the creation of HTML email templates that can be used to send marketing messages to a list of email addresses. These templates can include images, text, and interactive elements such as buttons and links. Both of these types of ads can be designed to be responsive, meaning that they will adjust to fit the screen size of the device they are being viewed on.