Let's say your company sends its customers quotes via email when a customer requests a quote. In the quote it has some fancy images, logos, and HTML/CSS to help sell the quote to the customer.
The problem: When you embed the HTML/CSS into an email, you are working at the mercy of the email provider to render the HTML/CSS correctly. First, different email providers will render your quote differently. Second, you have a subset of HTML/CSS to work with.
A workaround is to attach a PDF of the quote and also link the quote externally so a browser opens it. Doing it this way, you have the full arsenal of css/html that is available in a standard browser. The body of the email may look like this:
Your quote is attached. Additionally, you may <a href='url'>click here</a> to view your quote.
The question is, would there be a huge loss in views since people willing to open an attachment or click on an external link may be less? If possible, support your conclusion with data.