The IRS has resources for you; you don't have to reinvent the wheel.

https://www.irs.gov/e-file-providers/modernized-e-file-schemas-and-business-rules-for-individual-tax...

https://www.irs.gov/e-file-providers/tax-year-2019-valid-xml-schemas-and-business-rules-for-form-720...

https://www.irs.gov/e-file-providers/modernized-e-file-mef-stylesheets

"the client's customers would log in and use web-based software"

You want to Skin someone else's product? It seems you need to decide who is the Tax Preparer, first. And you know about Free File, right? Lacerte has an SDK available, but nothing from Intuit is meant for you to buy it as wholesale and then roll it out as your own product or portal.

Please note this is a peer end user community. You are not connecting with Intuit tech support directly.