The ELSE condition is not available in HTML templates.
Conditional output and variables are also supported in your project's HTML templates. You can use all the same include conditions in HTML templates that you can use in your topics with the Conditional Text tool. In addition to this there are also a number of special predefined conditional switches that are available in HTML templates only.
For details see Conditional output in the chapter on using HTML templates.
HTML template output conditions (reference list)