Current Result

https://github.com/zkoss/zk/blob/v9.5.0.1/zul/src/archive/web/zul/less/_reset.less
The file was copied from Bootstrap 2, an old version.
https://github.com/twbs/bootstrap/blob/v2.3.2/less/reset.less
 

Expected Result

Upgrade to 7.0.0 (supports IE9)
https://github.com/necolas/normalize.css/blob/7.0.0/normalize.css

Or use bootstrap's normalize.less + print.less
https://github.com/twbs/bootstrap/blob/v3.4.1/less/normalize.less
https://github.com/twbs/bootstrap/blob/v3.4.1/less/print.less

Or consider Reboot CSS used in Bootstrap 4
https://github.com/twbs/bootstrap/blob/v4.5.3/scss/_reboot.scss
 

Debug Information

button,
input,
select,
textarea {
  margin: 0;
  font-size: 100%;
  vertical-align: middle;
}

The vertical-align: middle; breaks third-party components, such as the checkbox of ag-Grid.