"However, in the 108th Congress, the House amended clause 8(b) of rule I(11) to require the Speaker to provide to the Clerk a list of Members to serve as Speaker pro tempore in the case of death or incapacity of the Speaker. Although the need to use this authority has not yet arisen, were a Speaker to die or become incapacitated during his or her term of office, a designated Speaker pro tempore, pursuant to this rule, would preside over the election of a new Speaker."
Precedents in various formats: https://www.govinfo.gov/collection/precedents-of-the-house?path=/gpo/Precedents%20of%20the%20U.S.%20House%20of%20Representatives/000-Precedents%20of%20the%20U.S.%20House%20of%20Representatives%20(2017%20series)
Current PDF version: https://www.govinfo.gov/content/pkg/GPO-HPREC-PRECEDENTS-V1/pdf/GPO-HPREC-PRECEDENTS-V1.pdf