[web][landing] Fix miscellaneous CSS issues

Summary:
Ran the code analysis tool in IDE which surfaced some misc CSS issues. I'll label each issue inline.

Depends on D4338

Test Plan: NA, careful reading

Reviewers: palys-swm, def-au1t, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4339