Update HTML tests.

This includes a new structural diff which is more robust, as it handles
changes like different attribute order.

1 files changed, 1 insertions, 0 deletions

diff --git a/tox.ini b/tox.ini
index dea9ee41..a6b993bf 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,7 @@ deps =
     pytest
     pytest-cov
     wcag-contrast-ratio
+    lxml
 commands = pytest {posargs}