Navigate to 'Templates' --> 'Web'. Then press 'Add Template'.
A Web Template editor opens. Please fill in the form 'preset'.
Create a 'Name'
Fill in a 'Tag' (is optional)
Choose a 'Master page'
Choose a branded 'InDesign Library'
Choose a 'Format ruleset'
Please fill in the form 'sizes'.
Give up the 'Width and Height'
Give up the 'Margins'
Then click on 'Save'.
Your created Web template has been added to the general Web templates screen.
