Client side HTML5 validation fails partially

I'm trying to amend server-side validation with HTML5 client-side validation. I ask the field for its regular expression and use it as a pattern for the input element:

#set($class = 'App.Bibitem.Code.BibitemClass')
#set ($xclass = $xwiki.getDocument($class).xWikiClass)
#set ($discard = $doc.use($class))
(% class="xform" %)(((
  #if ($xcontext.action == 'edit')
    <input type="hidden" name="xvalidate" value="1" />
    #set($field = "bib_o_label")
    #set ($definition = $xclass.get($field))
    ; <label for="${class}_0_${field}">$escapetool.xml($doc.displayPrettyName($field))</label>
    #set ($regexp = $definition.getValue('validationRegExp').substring(1))
    #set ($length = $regexp.length() - 1)
    #set ($regexp = $regexp.substring(0, $length))
    #set ($value = $doc.display($field, 'view'))
    : <input id="${class}_0_${field}" name="${class}_0_${field}" size="30" type="text" value="$value" required pattern="$regexp">
    ...

With XWiki 10.7 I see the following behavior when editing a page with a wrong input:

When pressing the Preview button, the page stays in edit mode and displays a tooltip "Please match the requested format.". Fine. That what I would expect.

When pressing the Save button, the page stays in edit mode and displays a bottom line error "Failed to save the page. Reason: Server not responding".

When pressing Save & View a freshly created page turns grey and displays the bottom line message "Saving... 0%" forever. Bad. A modified page stays in edit mode and displays a tooltip "Please match the requested format.". Fine.