The legal arguments
What Article 23 says:
Hong Kong shall enact laws on its own to prohibit any act of treason, secession, sedition, subversion against the Central People's government, or theft of state secrets, to prohibit foreign political organisations or bodies from conducting political activities in the region, and to prohibit political organisations or bodies of the region from establishing ties with foreign political organisations or bodies.
New legislation must conform to International Convenant on Civil and Political Rights and relevant Basic Law provisions;
Pure expression of opinion should not be criminalised; and
Secretary for justice's consent must be obtained for prosecutions under Article 23.
On sedition, an intention to incite violence or create public disorder against the 'constituted' authority has to be shown.
On treason, no objection to amending the Crimes Ordinance to reflect sovereignty change. Only action involving or likely to lead to violence to be liable to prosecution.
Regarding theft of state secrets, review Official Secret Ordinance to keep it in line with Johannesburg Principles on Restrictions on Freedom of Information.
On secession, the Bar says actual secessionist activity is covered under other laws.