Congress’ official tax scorekeeper released its explanation of the 2017 tax law, providing a window into the possible contents of future legislation fixing errors in the law.

The 457-page document from the Joint Committee on Taxation, known as the “blue book,” is Congress’ official list and explanation of all the tax changes that became law last year. The document, which is prepared by the JCT in consultation with the tax-writing committees in Congress, is used by the Treasury Department, Internal...