fixed typo on lesson 2.
prepare for lesson4.
fix typo and oversize problem.
Merge remote-tracking branch 'upstream/gh-pages' into gh-pages
prepare lesson 5 and 6.
add completed screenshot for ecardly.
add homework feedback and move html validator to earlier.
add the float and position demos.
Merge remote-tracking branch 'upstream/gh-pages' into gh-pages