I prepared a PR for an EAPI bump.
It also contains an update for the HOMEPAGE. I haven't seen the homepage as used in -r5, but it is no longer available and one gets rerouted to www.iso.org. I looked there for SGML and found a page for the ISO 8879:1986 which I hope is a useful homepage for this: https://www.iso.org/standard/16387.html
Feel free to give feedback.
The bug has been closed via the following commit(s):
Author: Bernd Waibel <firstname.lastname@example.org>
AuthorDate: 2017-11-25 17:01:39 +0000
Commit: Patrice Clement <email@example.com>
CommitDate: 2017-12-12 20:54:54 +0000
app-text/sgml-common: EAPI 6 bump and update HOMPAGE.
Package-Manager: Portage-2.3.16, Repoman-2.3.6
.../files/sgml-common-0.6.3-prefix.patch | 8 +--
app-text/sgml-common/sgml-common-0.6.3-r6.ebuild | 79 ++++++++++++++++++++++
2 files changed, 83 insertions(+), 4 deletions(-)