While researching issue #1551, I came across some minor problems.
When a spreadsheet does not have a title, which is often the case for spreadsheets created with Excel (note that this is not the case for spreadsheets created with PhpSpreadsheet), if you try to save it as Html, it throws an exception. It will now use the sheet title of the active sheet as a title in this case.
When writing an Html spreadsheet using `useInlineCss(true)`, gridlines are not handled properly. This is addressed by adding `class=gridlines gridlinesp` to the cell's `td` tag, and by suppressing any border attributes which would be styled as `none #000000`. It would be unusual to turn off gridlines for specific cells, but that can still be accomplished by using `Border::BORDER_NONE` in conjunction with any color other than `#000000` - see new test `testHideSomeGridlines`.
oleibman