code: public static @Nonnull FormValidation aggregate(@Nonnull Collection<FormValidation> validations) {
          if (validations == null || validations.isEmpty()) return FormValidation.ok();
  
          if (validations.size() == 1) return validations.iterator().next();
  
          final StringBuilder sb = new StringBuilder("<ul style='list-style-type: none; padding-left: 0; margin: 0'>");
          FormValidation.Kind worst = Kind.OK;
          for (FormValidation validation: validations) {
              sb.append("<li>").append(validation.renderHtml()).append("</li>");
  
              if (validation.kind.ordinal() > worst.ordinal()) {
                  worst = validation.kind;
              }
          }
          sb.append("</ul>");
  
          return respond(worst, sb.toString());
      }
docs: Aggregate multiple validations into one.
  
  @return Validation of the least successful kind aggregating all child messages.
  @since 1.590