My custom approach to Aurelia's validation view strategy. It's closely based on the twbootstrap-view-strategy.js from within aurelia/validation.

custom-validation-view-strategy.js

import {ValidationViewStrategy} from 'aurelia-validation/validation-view-strategy';

export class CustomValidationViewStrategyBase extends ValidationViewStrategy {

  constructor(containerClass, containerSuccessClass, containerErrorClass, appendClass, appendActiveClass) {
    super();
    this.containerClass = containerClass; // The class for the container.
    this.containerErrorClass = containerErrorClass; // What's the error class for the container
    this.containerSuccessClass = containerSuccessClass; // What's the error class for the container
    this.appendClass = appendClass; // Where we're appending the message
    this.appendActiveClass = appendActiveClass; // The class that activates the error block where we're appending the message.
  }

  searchForValidationContainer(currentElement, currentDepth) {

    if (currentDepth === 5) {
      return null;
    }

    if (currentElement.classList && currentElement.classList.contains(this.containerClass)) {
      return currentElement;
    }

    return this.searchForValidationContainer(currentElement.parentNode, 1 + currentDepth);
  }

  searchForAppendClass(currentElement, currentDepth) {

    if (currentDepth === 5) {
      return;
    }
    if (currentElement.classList && currentElement.classList.contains(this.appendClass)) {
      return currentElement;
    }

    let childElem = null;
    for (let i = 0; i < currentElement.children.length; i++) {
      let elem = this.searchForAppendClass(currentElement.children[i], 1 + currentDepth);
      if (elem) { childElem = elem; break; }
    }

    if (childElem) { return childElem; }
  }

  appendUIVisuals(validationProperty, currentElement) {

    let validationContainer = this.searchForValidationContainer(currentElement, 0);
    if (validationContainer === null) {
      return;
    }

    let appendElement = this.searchForAppendClass(validationContainer, 0);

    if (!appendElement) {

      let currentElementsParent = currentElement.parentNode;

      appendElement = document.createElement('p');
      appendElement.classList.add(this.appendClass);

      currentElementsParent.appendChild(appendElement);
    }

    if (validationProperty && validationProperty.isDirty) {
      if (validationProperty.isValid) {
        appendElement.textContent = '';
        appendElement.classList.remove(this.appendActiveClass);
        validationContainer.classList.remove(this.containerErrorClass);
        validationContainer.classList.add(this.containerSuccessClass);
      } else {
        appendElement.textContent = validationProperty.message;
        appendElement.classList.add(this.appendActiveClass);
        validationContainer.classList.add(this.containerErrorClass);
        validationContainer.classList.remove(this.containerSuccessClass);
      }
    } else {
      appendElement.textContent = '';
      appendElement.classList.remove(this.appendActiveClass);
      validationContainer.classList.remove(this.containerErrorClass);
      validationContainer.classList.remove(this.containerSuccessClass);
    }

  }

  prepareElement(validationProperty, element) {
    this.appendUIVisuals(null, element);
  }

  updateElement(validationProperty, element) {
    this.appendUIVisuals(validationProperty, element);
  }

}

export class CustomValidationViewStrategy { }
CustomValidationViewStrategy.Bootstrap = new CustomValidationViewStrategyBase('form-group', 'has-success', 'has-error', 'help-block-error', 'help-block-error-show');


// Activate by adding the config to you main.js
// import {CustomValidationViewStrategy} from 'custom-validation-view-strategy.js';
// .plugin('aurelia-validation', (config) => config.useViewStrategy(CustomValidationViewStrategy.Bootstrap))

// You can easily define your own in your main.js file, e.g.
// import {CustomValidationViewStrategyBase} from 'custom-validation-view-strategy.js';
// .plugin('aurelia-validation', (config) => config.useViewStrategy(new CustomValidationViewStrategyBase('form-group', 'has-success', 'has-error', 'help-block', 'help-block-error')))